Here is the argument that I used. I could not say something like "sex is only for procreation" because that's something stated in the bible and we couldn't use the bible to fall back on. Being a philosophy class we had to come up with a valid argument in which to claim that necrophilia is wrong.

Obviously no harm is done to the person because they are dead. They cannot object to any actions occuring because they are dead. So it's clear that no harm is done to the person. If I came home one day and saw someone keying up my car I would have the right to call the police and have the person arrested. The car is my property. Once somone dies they're body becomes incharge of the family. They own the body, hence they have to set up funeral arrangements and so forth. So if I walked in on someone having sex with my shoe, that is just was wrong as someone having sex with a body that I now own. As silly as this sounds, this was the best argument in class and the professor agreed that it was a valid point. So... if a familyless bum dies in the middle of nowhere... sex is on necrophiliacs.

One last note. You've be surprised how many morticians are closest necrophiliacs. I talked to a director of a forenics school.... it's sad really.