According to this report of BBC (link):

  1. It is estimated that at least 55% of British Pakistanis married their cousins. And now their 2nd generation is also marrying their cousins. 
  2. According to statistics, British Pakistanis are 13 times more likely to have children with genetic disorders than the general population of Britain. 
  3. In Birmingham city, one in ten of all children born to first cousins either dies in infancy or goes on to develop serious disability as a result of a recessive genetic disorder.
  4. Pakistani community account for just over 3% of all births in Britain, but they make 33% of all British children with such disabilities. (This means if half of British Pakistanis have cousin marriages, then 1.5% population of Britishers with first cousin marriages have 33% of all British children with genetic disorders)

And a recent scientific Study (which was done even on bigger scale) reported (link) that this trend of cousin marriages has even increased further in British Pakistani community in the recent years.

According to the DAWN Newspaper (Pakistan):

  1. The Universities of Bradford and Leeds together did a study in Pakistan. According to this study, 77% of all disabled Pakistani children were born due to cousin marriages. 
  2. In the city of Charsadda (Pakistan), 90% of people married their cousins. And as a result, every third family has a disabled child with genetic disorders for the last 40 years. People of this city know the reason for this too, but still religion of Islam make them to continue this practice of marrying the cousins. 

According to the "The News" newspaper (Pakistan):

  1. Consin marriages in Pakistan are 73 percent and as a result, the country has the largest community of deaf children in the whole world.

  2. The number of people suffering from genetic diseases in the country is estimated between 14-16 million while 1.6 million mutations have been found in Pakistan.

Marriage with Cousins is also incest

The rate of sharing of common genes among close relatives is as under:

  1. Parents/Children: 50% common genes
  2. Siblings: 50% common genes
  3. Uncle/Niece: 25% common genes
  4. First Cousins: 12.5% common genes
  5. Second Cousins: 3.125% common genes

Thus, cousin marriages are prohibited in many states of US, and it is even considered as a Criminal Offence and punishment is given for such an act (link). The reason for this is simple i.e. First cousin marriage is also an incest relationship.

Why cousin marriages have become a nightmare for Muslims in present era as compared to the past era?

In the past centuries, cousin marriages didn't become such a headache for Muslims, while:

  • Muslims were aggressively attacking other nations, capturing their women, and raping them and having a lot of children from them.
  • They were also buying a lot of slave women directly from the slave markets, and their Harems were full of dozens of slave women.  
  • And then there were many widows present in society due to constant wars. Thus, Muslims were having multiple wives too in polygamy, and many of them were not cousins.  

All that contributed to the fact that Muslims were not hit hard from the diseases of cousin marriages as a society in the past.

But the situation has changed in the present century, while:

  • Slavery became non-existent.
  • Muslims are weak and unable to do wage offensive wars today, and unable to capture women of non-Muslims and to rape them. 
  • Polygamy almost ended and now Muslims have only one chance to pick their life partners, and they are making a mistake of picking their cousin sisters as their first and the only choice. 

That is why, cousin marriages have become a nightmare for Muslims in the present era. 

Moreover, Islamic teaching of Hijab and so-called Islamic Modesty has created a very isolated family system, where young boys and girls get chance to interact with each other only in case if they are cousins (i.e. it is almost impossible for Muslim boys to come in contact with girls who are stranger to them). Thus, in maximum cases, cousin boys and girls are falling in love with each other. Thus, this closed family system due to Islamic Hijab and so-called Islamic Modesty is also a culprit in increase of cousin marriages in Islamic societies. Of course, a Muslim girl finds it more convenient to marry a man who she knows (i.e. her cousin brother) instead of marrying a total stranger who she absolutely doesn't know. 

Strange Islam: Breastfeeding milk makes a person unlawful to marry, but cousin incest marriages are ok

Islam says if any woman has breastfed a male child (or an adult man) 5 times, then the daughter of that woman becomes your "sister" and it becomes unlawful to marry her. 

But modern science proves that breastfeeding a child (or an adult) 5 times (or even hundreds of times) makes no changes in the genes of a child, and no genetic disorders occur in the progeny if he marries the daughter of that woman. 

It was only a tradition of time of ignorance of Arab society, and Muhammad kept on following it, while there exist no All-Knowing Allah in the heavens who could have stopped Muhammad from making this mistake. 

Please ponder upon it, is it not the same concept which is present in Hinduism too, where they declare cows to be their mothers while they drank their milk, as it has been written in their religious books?

1st excuse by Islam apologist: We don't accept science as it changes

Islam apologists deceive Muslims (and non-Muslims) by telling them that science cannot be trusted, as it changes with time. They intentionally mix up "scientific hypothesis" with "scientific facts/laws". 

Yes, there are chances that a scientific hypothesis changes later (or proves to be wrong all together) in the light of experiments, but there are some things which have already reached to the level of being "Scientific Facts/Laws", and they are not going to change. For example, it is a scientific fact that earth is not flat, but a sphere, and this scientific fact is not going to change. Similarly, it has now become a scientific fact that cousin marriages are a cause of disabled children with genetic disorders, and this fact is not going to change.

Unfortunately, "Scientific Facts" still have no value in front of "Blind Belief" of religious brainwashing. 

2nd excuse by Islam apologists: Spreading fabricated Ahadith 

Some Islam apologists are spreading Ahadith, which discourage marrying close relatives. But these Ahadith are fabricated and not found in any Hadith books of Muslims. Let us quote from a Muslim website itself for this deception (link):

Question: Did the Prophet (peace be upon him) discourage marrying relatives (ie. cousins) even though it is lawful?

Answer:

The short answer is that there are certain narrations that discourage marrying cousins, yet experts of hadith verification have determined them to be extremely weak or fabricated.

The Narrations in Question

– “Do not marry within the family [i.e., cousins], as that leads to children that are thin and weak.”

– “Do not marry within the family [i.e., cousins], since the child would be born thin and weak.”

– “Marry outside the family, lest the offspring be thin and weak.” [Ibn Hajar, Talkhis al-Habir]

Regarding these and similar narrations, the 7th century hadith specialist Ibn Salah said, “I found no reliable basis for them.” Many eminent hadith masters mentioned his statement and concurred, such as Ibn Hajar al-Asqalani, Ibn Mulaqqin, and others. [Ibn Hajar, Talkhis al-Habir; Ibn Mulaqqin, Khulasat al-Badr]

Taj al-Subki said regarding these narrations, “I found no chain of transmission (isnad) for them.” [Subki, Ahadith al-Ihya Alati La Asla Laha]

Hence it can be concluded that these narrations — as statements of the Prophet (peace and blessings be upon him) — are fabricated.

After mentioning Ibn Salah’s verdict, Hafiz Iraqi does state that it was rather Sayyidina Umar who made such a statement, specifically in response to a certain family/tribe whose offspring were thin and weak. He said, “You all have become thin and weak, so marry outside the family.” [Iraqi, Takhrij Ahadith al-Ihya’]

So, there is no Hadith present from Muhammad which discourages marrying within family. 

Actually, contrary to it:

  • Muhammad himself fall in love with his first cousin Umm Hani, who was daughter of Abu Talib. But he refused to marry her with Muhammad as he was not only poor, but he had also learnt no skills to earn money, and thus Abu Talib didn't consider Muhammad to be noble enough to marry his daughter to him (link).
  • Then Muhammad married to Zainab bint Jahsh, who was his cousin sister. 

As far as the saying of Umar is concerned, then firstly, he was neither Allah nor a prophet and his words are not part of Sharia. 

Secondly, it was a common observation of even the people of old times that marrying a strong woman bore stronger progeny, while marrying a weak woman bore weaker children. Thus, when Umar saw that the tribe of al-Sa'ib became weaker physically, then Umar suggested particularly only to them to marry in people who were more intelligent and stronger than them (i.e. if the people of al-Sa'ib were stronger, then Umar would not have made this suggestion to them). Therefore, it was only an exceptional case, and absolutely not a part of Islamic Sharia. And there exists no general recommendation from Sharia to marry outside the family. 

Moreover, Islam apologists also present the following (link):

عن الشافعي قال: أيما أهل بيت لم يخرج نساؤهم إلى رجال غيرهم كان في أولادهم حمق. 

Al-Shafi’i said, “Whenever the people of a household do not allow their women to marry men outside of their line, there will be fools among their children.”

Again, this saying of al-Shafi'i is not a part of Sharia, but it was a common observation of people of even that era that marrying within family only bring diseaseas to the progeny. Thus, even Ibn Hajar al-Asqallani (another celebrated Muslim scholar) refuted it to be a part of Islamic Sharia, and wrote (link):

وأما قول بعض الشافعية يستحب أن لا تكون المرأة ذات قرابة قريبة فإن كان مستندا إلى الخبر فلا أصل له أو إلى التجربة وهو أن الغالب ان الولد بين القريبين يكون أحمق

(Ibn Hajar recorded the saying of al-Shafii and said) ... there is no basis of this saying (in Islamic Sharia), although it can be said on the basis of EXPERIENCE that often children start becoming fools due to marriages in the close relatives.

So, there exists 'no recommendation' in Islamic Sharia to avoid incest marriages with cousins. 

Conclusion: 

There is no All-Knowing Allah present in the heavens, but it was Muhammad himself who was making this religion at his own. And he followed the traditions of Arabs of time of ignorance of that time. Thus, all the innocent children, who were born with genetic disorders due to cousin marriages, then Muhammad is also responsible for their sufferings. 

An excuse from Islam apologist: 

An Islam apologist objected and wrote:

"Muslims were aggressively attacking other nations, capturing their women, and raping them and having a lot of children from them." - you really think common Muslims were going around raping women? Do you think that common Christians were also going around raping women during the crusade or imperial era? No, obviously not. Most were just living in their own community for generations.

Answer:

Yes, Muslims were going around and raping the women. There were a lot of Jihadi wars, and almost half of the male population served in armies and fought wars. Families had many children at that time and some of them did join the army in order to get easy money and war booties and slave women and children.

And we are neither Christians nor their defender that you ask this question to us.

Nevertheless, evidence is there that Christians were not involved in cousin marriges at that mass level as Muslims were involved (while Church opposed it).

Secondly, slavery was also bad in Christianity, nevertheless, the rape of innocent prisoner women/slave women was 100s of times worst in Islam. It was due to the reason the Muhammad adopted the laws of Arab culture regarding rape of prisoner/slave women and left the Sharia of Moses which was much more lenient as compared to the Arab culture.

Allah/Muhammad not only allowed the owner to rape the slave-girl, but also to hand her over to other men to rape her one by one in a "temporary sexual relationship" 

Allowing a Muslim owner to rape a prisoner/slave-women was itself a great crime of Allah (i.e. Muhammad) against the humanity.
But Allah went even further in cruelty against the prisoner/slave-girl, and Allah dishonored the poor slave-girl in one of the worst possible ways by letting her to be raped by multiple Muslim men one by one.
Allah made it Halal for the Muslim owner that after fulfilling his Lust in Shia Mut'a type "Temporary Sexual Relationship), if he got bored, then he could hand her over to one of his brothers (or even to any of his slave). And after all of his brothers (/or slaves) had raped her one by one and fulfilled their lust and got bored, then she could be sold to the 2nd master, who is again going to rape her, and then sell her to the 3rd master .... and thus, this cycle of rape continued for the poor slave-girl.
Sahih Muslim, Kitab-ul-Nikah (link), Sahih Bukhari, Kitab-ul-Qadr (link), Sahih Bukhari, Kitab-ul-Tauheed (link):
0 Abu Sa'id al-Khadri said: We went out with Allah's Messenger on the expedition to the Bi'l-Mustaliq and took captive some excellent Arab women; and we desired (to have sex with) them, for we were suffering from the absence of our wives, (but at the same time) we also desired good ransom money for them by selling them). So we decided to have sexual intercourse with them but by observing 'azl (i.e. withdrawing the male sexual organ before emission of semen to avoid-conception so that they don’t become pregnant). But then we said: We are doing an act whereas Allah's Messenger is amongst us; why not ask him? So we asked Allah's Messenger, and he said: (Yes, it is allowed, but) it does not matter if you do it or not, while if any soul has to be born up to the Day of Resurrection, then it will be born.

Allah/Muhammad allowed the master to snatch away the wife of his slave, and to start raping her, and thus destroying the whole slave family. 

Sahih Bukhari, Book of Marriage (link):

وَقَالَ أَنَسٌ: {وَالْمُحْصَنَاتُ مِنَ النِّسَاءِذَوَاتُ الأَزْوَاجِ الْحَرَائِرُ حَرَامٌ إِلاَّ مَا مَلَكَتْ أَيْمَانُكُمْ لاَ يَرَى بَأْسًا أَنْ يَنْزِعَ الرَّجُلُ جَارِيَتَهُ مِنْ عَبْدِهِ.
Companion Anas Ibn Malik said: The meaning of the verse (وَالْمُحْصَنَاتُ مِنَ النِّسَاءِ Surah Nisa) is this that if a slave girl of any person is in the marriage of his slave man, then owner could take that woman back for himself (to have sex with her) from his slave man.

Unbelievable!

Even Judaism/Christianity dealt with slave women much better than Islam

There are verses in Bible which tell the Moses and other prophets killed the men, and took the women for themselves (just as Muhammad killed all the captive men, along with 12 years old male children, and took all the women as slaves). 

But after the women have already been captured, then Bible treats them in much more humanely and with respect as compared to Allah/Muhammad. 

Bible didn't allow the owners to rape and selling of the captive/slave-women again and again after temporary sexual relationship like Islam, but they stipulated that:

  • An owner is allowed only to "marry" the captive/slave-woman.

  • After marriage, the status of captive/slave-woman became like a permanent wife.

  • And such slave-woman could not be handed over to the brother or slave as a sex object, and could not be sold to another owner after temporary sexual relationship, but she becomes automatically a free woman. 

  • And off course bible didn't allow the owner to snatch away the wife of his slave, and to start her rape, and thus to destroy the whole family of his slave. 

Bible, Deuteronomy, Chap 21 (link):

Marrying a Captive Woman
10 When you go to war against your enemies and the Lord your God delivers them into your hands and you take captives, 11 if you notice among the captives a beautiful woman and are attracted to her, you may take her as your wife. 12 Bring her into your home and have her shave her head, trim her nails 13 and put aside the clothes she was wearing when captured. After she has lived in your house and mourned her father and mother for a full month, then you may go to her and be her husband and she shall be your wife. 14 If you are not pleased with her, let her go wherever she wishes. You must not sell her or treat her as a slave, since you have dishonored her.