Death is important and means many significant things to people everywhere, always has, always will. People are accustomed to symbolizing the loss of a loved one through ritual, physical manifestation such as monuments, and other ways which help them solidify a memory, for lack of better words.
It might be lame an explanation to some, others may think it’s unnecessary to treat the dead with the respect or significance we do, but that’s what it is, generally. So of course fucking a corpse is seen as an extreme lack of respect, as it’s akin to desecration, if not worse.
I’m willing to believe that some necrophiliacs might believe that to have sex with the dead is a form of contact or something spiritual akin to putting up headstones or wrapping mummies, and in their heads it’s not anything offending. After all, while most cultures in the world follow the same mindframe when it comes to death, the methods to represent this are various in form and shape. (Official or otherwise.)
However, there’s also the suggestion that necrophilia may be, or may be part of, a psychological disorder. If you’ve ever seen a corpse in real life, fresh or otherwise, it’s just not something that the human psyche is equipped to deal with when it comes to sex, culture/beliefs or not. But I’m just guessing at this point.
As to why it’s taboo, other than it being just nasty I mean, what I’m speaking of here highly deals with belief, and belief is an extremely strong component in what makes a human human, and people don’t like having their beliefs challenged, shattered, mocked or threatened, whether that’s with people on Fluther disputing God or some dude bangaranging a stiff. Nevertheless, if death didn’t exist, a hell of a lot of belief systems wouldn’t, either.