A dead body is an object not a "party". If you want to condemn the behavior you'll have to find a better justification than rape.
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.
Yes my whole point was that this is irrational and illogical thinking, and that by being tolerant of gays but not other sexual orientations you are merely conforming to what society accepts rather than being a logical and critical thinker or a truly tolerant person. I find it ridiculous that so many "accept" gays and pat themselves on the back for how tolerant and progressive they are when really they are just conforming their opinions based on whats currently accepted regardless of how hypocritical or illogical the line of thinking is.
People now think about people that have sex with dead bodies similarly to the way people used to feel about gays. We haven't become a nation of tolerance, we've just moved the goal posts and declared victory.