I don’t think it’s necessarily morally wrong to have sex with a dead person. Even if it’s intentional necrophilia and the person told you not to do it when she or he was still alive, they’d have no way of knowing you did it once they died unless, of course, there really is a such thing as life after death, which there almost definitely isn’t. In other words, what the person doesn’t know almost definitely won’t hurt them.
If the dead person’s parents were to walk in on you having sex with it, you would probably cause them emotional distress or “hurt”, but even that wouldn’t be wrong unless the body, which, for the sake of this example, is an adult, belongs to the parents in the way of private property. Aside from what the law might say, which means nothing in regard to my morality, I don’t know if that’s ever the case.