Whether you think it's rational or not, our society and many other have a thing about treating dead bodies with "respect" and necrophilia is a transgression of that taboo.  Many people are disturbed by anything they perceive as disrespectful being done to a corpse and it's going to cause them mental anguish if they find out that the corpse of a loved one has been "mistreated" in any way. 

Hell, many people refuse to allow autopsies to be conducted on corpses and won't donate organs - even when the decided clearly indicated that they wanted to be a donor - because they regard even dead human bodies as being somehow "sacred".  People generally regard murdering someone and mutilating their corpse as somehow being "worse" than simply killing them.  Society has a thing about how dead bodies should be treated.

There's clearly no harm being done to the corpse itself, but I don't think that the average person is going to react well to learning that someone fucked the corpse of their loved one - whether you believe that's rational or not is irrelevant, it's internally consistent if people hold the viewpoint that dead bodies should be treated with respect.