If you allow necrophilia if the person consented while alive then you have to allow cannibalism as well. Throw in bestiality because it really doesn't make sense to able to screw your sister or dead brother but not your sheep or pig. And hey I can screw my dead brother's pet cat after eating him for breakfast why can't I snort crystal meth.

Society has the right to say there are certain taboos that are not allowed and it does have the right to set limits on acceptable behavior. People are willing to allow hate speech laws but oppose laws that violate the social norms of virtually every human society in history. Those social norm evolved for a reason and it is very dangerous to think you can just toss them out at will.