It is one of the favorite argument of Muslims against forming a Godless Secular system.

They propagate this idea that without Allah's Sharia system, people will start raping their mothers and sisters and the dead bodies. 

Answer:

A non-religious society is fully capable of saving its members from any harm by using its intellect, commonsense and wisdom. They don't need any Allah for this purpose.

Basically, this act (i.e. raping the dead bodies of women/girls) happens very often in Islamic Pakistan due to the 'extreme Sexual Frustration' in the society, which is ultimately formed due to the unnatural restrictions upon the interaction with the opposite sex. 
Pakistani newspapers are full of such news that people take out the bodies of newly buried women/girls from the graves and then rape them. This disease increased so much that relatives of the deceased women/girls have to guard their graves for next many nights.

Please see this link and you will find tens of news about rape of dead women/girls in every city of Pakistan (in English). 

The foremost Reason for such behavior (like raping dead bodies or indulging in incest) is Sexual Frustration

First Solution:

By using our intellect, it was very easy for us to find out that the foremost reason for such behavior (like raping dead bodies) is Sexual Frustration. And we absolutely don't need Allah to find out the main cause of this problem.

After finding the reason, the 2nd step is finding the solution by using our intellect.

We removed the 'unnatural restrictions' upon the interaction of men and women, which helped to remove the sexual frustration from the society.

And even if someone desires such a thing after that too, then it is a sign that he is mentally ill and needs medical help.
 

Second Solution:

Moreover, in a non-religious society, another solution was also sought by using our intellect i.e. The whole society declared such an act (i.e. raping a dead body) to be a crime. We did it in order to save members of our society from any physical or psychological harm. It is exactly the same that some men in non-religious society can have tendencies towards taking drugs, but society as a whole declare it to be a crime in order to protect its member from being harmed.

We, as atheists, consider that NATURE is not perfect. Not all is black and white in our world. Of course, there exists few people in society with tendencies towards crimes and fetish Fantasies. And due to this imperfection of Nature, we have to find solutions by using our intellect, in order to deal with them.

If Muslims say that they don't rape dead people while their Allah prohibited it, then we tell them simply that we don't rape dead people while our society has collectively found this solution that raping a dead body is harmful and thus a crime.

And the results speak for themselves, where due to unnatural restrictions of interaction between men and women, all Islamic societies are suffering from extreme Sexual Frustration.

Not only Islamic Pakistani society is ahead in raping the dead bodies of women/girls, but they are also no.1 in watching porn films in the whole world.

Actually, not only porn films, but Pakistani Society is No. 1 in watching INCEST PORN (like sister fuck, mother fuck, daughter fuck, donkey fuck) in the whole world for the last 2 decades (according to google Trend).

Here are the links about Pakistanis being No. 1 in search of incest porn.

  1. Sister Rape

  2. Mother Rape

  3. Child Sex

  4. Donkey Rape

Tehrir Square (Egypt) Incident: 

This one video is enough to see the sexual frustration among the Muslim masses due to unnatural restrictions in name of Hijab and modesty. This is much more dangerous than the incident of Muslim men digging the graves and raping the dead women/girls.

 

Muslim Women abused and harassed by Muslim men during the most spiritual Hajj and 'Umra gatherings

There are thousands of stories, told by Muslim girls how they were harassed by Muslim men during Hajj and 'Umra, due to the combined gathering of women and men (link). Yes, this is only one instance where Muslim men and women have the combined gathering. 

What Muslims do during the gathering of Hajj, that we non-Muslims don't do even during the gathering of Discos, although we are doing music and dance in discos. Islam claims that music and dance are Haram while they bring humans towards the dark side of Satan. But contrary to the claim of Islam, the truth is that music and dance don't make us become such sexual frustrated animals as Muslims are during the Hajj gatherings.

Our intellect tells us that this same sexual frustration leads the men towards even raping dead bodies of women/girls in the next step.

Muslim Dilemma

Muslims can hit their wives and cannot say that hitting a wife is wrong because it is written in their book.

At least we atheists can discuss sleeping with a dead body and sex-slavery and all other issues from an ethical and disgust and harming and many other standpoints. Muslims cannot discuss ethics, they can only defend a set of outdated ethics while they themselves are not convinced by them.

Muslim Objection: Why then rape still happen in the West despite freedom of interaction between men and women?

A Muslim presented the following objection:

West is not repressing sex, yet rape and assault happen here as well. Why is that happening?

Answer:

While nothing is 100% PERFECT in this imperfect world.

We can use our intellect to solve the majority of issues, but making a 100% fault free perfect system is not possible.

Nevertheless, if we compare our non-religious system with the so-called Perfect Islamic System of the so-called 100% Perfect Allah, then we easily win in every field.

Muslim Objection: But few atheists in Western countries support the idea of allowing necrophilia

Muslim objection is:

There are some atheists in the western countries who support the idea of necrophilia, incest, pedophilia (some would argue Muslims debate this too) and bestiality too? 

Answer:

If some people in a non-religious society raise their voice in favor of such things, then it is known as "Freedom of Speech/Freedom to Criticize", which is a very healthy thing to have.

We know neither Nature is perfect nor human beings are perfect. There are human beings with widely different views and tendencies. They are all allowed to verbally present their views, but in the end they all have to follow the collective decision of society (i.e. the law).

Through giving them the freedom to express their views, we are helping ourselves to understand the problems of our society in a much better way. And if we can understand the problems properly, only then can we find out the best possible solution for this too.

Evolution in our behavior and laws comes through this constant freedom of expression and criticism. We may make mistakes, but we are also fully capable of reforming ourselves after looking at the results of our mistakes and wrong decisions. 

Reality is, regardless of what Muslim apologists like to portray in the west, it’s the Muslim world that has a harder time dealing with things.

They may try to say that without Islam we would be doing all these things but (while you might find someone arguing for these things) the west has done a better job making them taboo (i.e. less widespread) ... Please see how Pakistani Muslims are also at top of list in search of "Donkey Rape" along with "Incest" related search. 

Conclusion:

Like Lawrence Krauss said, “reason and empathy”, and we will solve all problems sooner or later. 

Reminder: Islam doesn't say anything about necrophilia, it's up to the opinions of scholars

Yes, as the title says. In Islam there is no verse nor a Hadith nor an Athar from a companion or a Tābiʾi.

It's left to the fallible human agents after Muhammad who's infallible -from the POV of Islam- to decide on this matter that seems to be the go-to for Muslims when debating atheists.

The scholars of Fiqh agreed on the illegality of having sex with a dead person. However they're split to two opinions on the punishment of the person who does it.

In the Kuwaiti Encyclopedia of Jurisprudence, the two views are:

  • Hanafis, Shāfi'is (in the more sound view) and Hanbalis (in the accepted view) say that there is no Hadd for the person committing necrophilia, that is because having sex with a dead person is like not having sex because it is (the sex) done with a wasted organ, and because having sex with the dead woman is undesirable, in fact, it's something that the [human] nature would have an aversion to and people would be disgusted by. So there is no need for establishing a Hadd as a deterrence. But a Ta'zir should be done on the person committing such act.

  • Mālikis, Shāfi'i (the opposing view) and some Hanbalis are on imposing the Hadd punishment on the doer, and this is the view of al-Awzā'i. They argued that it's because it's similar to having sex with a live woman, and because it's a graver sin for violating the sanctity of the dead (this is baseless). The Malikis excluded the dead wife and stated that there is no Hadd on the husband for having sex with his dead wife.

So, what are the reasons for the scholars?:

  • Appeal to disgust (yuck factor).

  • Appeal to nature.

  • Ad populum or appeal to the people.

  • Argument from analogy. Their Qiyas is broken and error-prone.

As you can see there is no evidence from the Quran or the Hadith that says anything about the matter, there is only the guesswork of the scholars. Even the Ta'zir punishment is subjective and baseless.

So, the Muslims have no basis for considering necrophilia immoral or haram, even their typical answer "because Allah/Prophet says so" doesn't work here. And even if it did, it won't be acceptable as long as their religion is nothing but unfounded claims.

PS: 

Muslims are too ashamed of this today, that when an Egyptian Preacher repeated the same fatwa that man can have sex with his dead wife, then he was prohibited to appear anymore on the TV. Here you can read about it.