After the death of Khadijah, Muhammad had about 16 (+/-) wives and slave women, but he got no children from any of them.

Sahih Muslim, Hadith 2771:

Anas reported that a person (a Coptic slave whose name was "Mabur" and he was the cousin of Maria al-Qibtiyya) was charged with fornication with the slave girl of Allah's Messenger (i.e. Maria al-Qibtiyya). Thereupon Allah's Messenger said to 'Ali: Go and strike his neck. 'Ali came to him and he found him in a well making his body cool. 'Ali said to him: Come out, and as he took hold of his hand and brought him out, he found that his sexual organ had been cut. Hadrat 'Ali refrained from striking his neck. He came to Allah's Apostle and said: Allah's Messenger, he has not even the sexual organ with him.

Imam Hakim recorded the following tradition in his book al-Mustadrak (link):

«عن عايشة قالت : اهديت مارية إلى رسول اللّه ومعها ابن عم لها . قالت : فوقع عليها وقعة فاستمرت حاملا . قالت : فعز لها عند ابن عمها . قالت : فقال اهل الافك والزور : «من حاجته إلى الولد ادعى ولد غيره» وكانت امّه قليلة اللبن فابتاعت له ضائنة لبون فكان يغذى بلبنها فحسن عليه لحمه . قالت عايشة : فدخل به علىّ النبى صلى الله عليه وآله وسلم ذات يوم فقال : «كيف ترين»؟ فقلت : من غذى بلحم الضأن يحسن لحمه . قال : «ولا الشبه» قالت : فحملنى ما يحمل النساء من الغيرة أن قلت : ما أرى شبها . قالت : وبلغ رسول اللّه ما يقول الناس فقال لعلى ... ».

Aisha said: " Maria" was presented to the prophet of Islam (as a slave woman) and her cousin (a coptic male slave) was with her. After a while Maria became pregnant. Upon that, the people started slandering that since he [the prophet of Islam] needed child, he related the son of that slave-man to himself. Since Maria, as a mother, didn’t have enough breast milk, they fed him by sheep 's milk that’s why he (the son Ibrahim) was fat. 'Aisha said: Once the prophet brought him to me and asked what I thought about him, I replied, "everyone fed by sheep 's milk will get fat." The holy prophet said doesn’t he look like me? Aisha said, "I jealously said "No"." and then the prophet heard of the untrue accusations of people toward Maria. Upon that the prophet sent Ali to kill her cousin (i.e. the Coptic slave) ...

 Therefore, when it came to the personal life of Muhammad, then:

• Muhammad didn’t give that Coptic slave a chance for a fair court trial. 
• He didn’t ask the people (who accused that Coptic slave) to bring the 4 witnesses for fornication.
• And Islamic punishment for fornication in the case of slaves is only 50 lashes. But Muhammad himself went against the Sharia and ordered to kill that slave. 

Isn't Muslim claim that Muhammad was the best example? Shouldn't the best example follow the rules, so that people themselves follow?

Moreover, all those Sahaba who made that false accusation of fornication against that slave and Mariyah should have been lashed 80 times and their testimony should have never been accepted in the court anymore.  It is called حد القذف in Islam. Muhammad did the same in the case of 'Aisha in the incident of Ifk (i.e. he lashed all those Sahaba who made a false accusation of fornication against 'Aisha). Please read our article IFK incident: A proof of Human Mistake in the Quranic revelation

And they wonder why honour killings still happen in Muslim countries. If the leader didn't follow the rules, then the followers probably wouldn't too. 

Muslims made up this excuse that perhaps Muhammad had already known that he didn’t have a sexual organ. But this excuse could not be accepted while even if we assume that he knew it, still he had only two options: 

• Firstly, if he had already known it, then he would have taken absolutely no action against that Coptic slave as he knew that he was innocent. 
• Secondly, and despite knowing the reality, even if he still wanted to take action, then he should have followed the rules and gone through the court trial. 

 Actually, the reality is this Muhammad was not only accusing that Coptic slave of fornication, but he was indirectly also accusing Mariya too of fornication. Therefore, even after Ali had told Muhammad that the Coptic slave didn't have a sex organ, still Muhammad kept on doubting that Mariyah fornicated, and thus Muhammad kept on doubting the parentage of Ibrahim. In the end, Jibrael had to come to him and confirm that Ibrahim was his son. 

Ibn Kathir recorded the following tradition (link):

عن أنس قال‏:‏ لما ولدت مارية إبراهيم كاد أن يقع في النَّبيّ صلَّى الله عليه وسلَّم منه شيء حتَّى نزل جبريل عليه السلام فقال‏:‏ ‏(‏‏(‏السلام عليك يا أبا إبراهيم‏)‏‏)‏‏.
Anas said, when Ibrahim was born to Maria, then Muhammad became doubtful if Ibrahim was really his son or not. Upon that angel Jibrael came to him and said: Peace be upon you, O the Father of Ibrahim (i.e. confirmed to him that Ibrahim was indeed his son). 

 Actually, there was no Jibrael, and Muhammad had to use the name of revelation and Jibrael in order to keep the Sahaba quiet. 

Moreover, this tradition is also proof that Muhammad didn't know previously if the Coptic slave had a sexual organ or not, while he kept on doubting Mariyah even after Ali told him that the Coptic slave didn't fornicate with Mariya due to his inability. Therefore, the prime suspect in eyes of Muhammad was Mariyah.  

These rumours continued till the time of the death of Ibrahim. Thus, on his death, Muhammad was unintentionally claiming that Ibrahim was indeed his son. 

Sahih Muslim:

فلما توفى إبراهيم قال رسول الله صلى الله عليه وسلم ان إبراهيم ابني وانه مات في الثدي وان له لظئرين تكملان رضاعه في الجنة

"...  Ibrahim is (indeed) my son and he dies as a suckling babe. He now has two foster-mothers who would complete his suckling period in Paradise." 

People doubted that Muhammad had fertility problems, while according to Muslim scholars, he married Kadhija who was 40 years old, and all his 6 children were from her except Ibrahim. So how come he had only "one" child from dozens of young women and 6 from 1 old woman?

The Huge Scandal of Mariya stopped Muhammad from any more adventures of marrying other women

The answer lies in this huge scandal of Mariyah (his slave girl).

While Muhammad failed to make any other of his existing 12 wives bear a child for the last decade. Thus, people considered Muhammad to be infertile. In this situation, when Mariya became pregnant, then rumours arose that Mariya had a hidden sexual relationship with her cousin (the Coptic slave).

Even Muhammad himself was considered to be infertile, and thus believed in those allegations, and he ordered the killing of that Coptic slave without any court trial or witnesses. 

And even after it became clear that the Coptic slave didn't have any sexual organs, still Muhammad kept on suspecting Mariya of illegal sex. But he was unable to punish Mariya without the witnesses. And then people started making even more rumours about it. In order to make their mouths shut, Muhammad then used revelation and claimed that Jibrael came and confirmed that Mariya was pregnant from him.

Thus, it seems that this huge scandal of the illegal sexual relationship of Mariya ended Muhammad's journey of marrying any other woman, while people had already been raising questions about Muhammad's fertility.

Muslim Excuse: This was the punishment for Insulting Muhammad for "Entering his private chamber"

Islam apologist writes (link):

The actual wording of the hadith says أم ولد which means that it refers to Mariyah al-Qibtiyyah, the mother of the Prophet's son Ibrahim. The accused was a person who had accompanied her from Egypt, sometimes claimed to be her cousin. He would visit her too often and people speculated that he was attracted to her and had relations with her. It is inferable that he persisted even though he knew of the rumors about him and had previously been warned about his inappropriate behavior ... Entering in the private chamber of the Prophet's Umm al-Walad and being alone with her repeatedly would be basis enough for a Ta'zir punishment and that much would have been proven through the reports of witnesses who had informed the Prophet in the first place. Insult to the prophet is punishable by death ... The evidence criterion for Insult and Ta'zir is different from that of fornication.

Answer:

If Islam apologists are true that it was the Ta'zir punishment for insulting Muhammad by entering his chamber (and not fornication), why then Ali didn't kill him for this insult in the end? His so-called crime of insulting Muhammad by entering the Chamber of Muhammad despite so-called warnings didn't end with him not having the sexual organ. 

Giving no punishment to that slave person is proof enough that it had nothing to do with the so-called accusation of "insult" by the Islam apologists, but it had to do with the accusation of fornication. 

It is simply that there existed no so-called "previous warning" against him, and Islam apologists fabricated this accusation against him on their own.

Actually, these Islam apologists don't realize that by claiming that that slave man used to enter in the chamber with Mariyah al-Qubtiyya, they are in fact insulting their Prophet themselves by accusing him of indecent behaviour, where he let that person visit Mariyah and didn't stop him, or to Mariyah to meet each other in his chamber. 

Therefore, if they were really meeting each other in his chamber, then it was the duty of Muhammad and he should have forbidden him from visiting her. And if they claim prophet Muhammad didn't stop them from meeting each other in his chamber while Mariyah was a slave, then how come that became an act of insult to Muhammad in the case of that slave man? After all, then it is Halal in Sharia to meet the slave girls of other men in the chambers and Muhammad didn't stop him to visit Mariyah. 

Conclusion:

Have you heard the story of the lamb and the wolf, where the cunning wolf wanted to eat the lamb by making false accusations against him? Wolf first blamed him for making his drinking water dirty, but the water was running from the wolf to the lamb. Upon that wolf made the next accusation that the lamb abused him last year, but the lamb was not born last year. Upon that wolf came up with the next accusation that then it was his father who abused him the last year and in order to teach this family a lesson, he had to eat that lamb. 

These Islamic apologists are the same as that cunning wolf, who had to make false accusations again and again against the innocents in order to achieve their evil plans.