I didn't claim there is an easy way to determine coersion.
I think when we're talking about child abuse, we almost never can determine
the lack of coersion with certanty and so these things have definitely to be
forbidden and get punished very hard;
but talking about necrophilia - I think it's easy to determine if someone
gets coerced.
I find this topic really disgusting, but if two people agree on this (one of
them when he was still alive) and no one gets forced or harmed - why should
it be illegal?
But yeah, this is a topic for the troll hole.
I just wanted to correct the very incomplete quote of stallman.