Absolutely true - what is wrong for a necrophiliac to do what he wants with a dead body.
We might find it disgusting and appaling, but that is merely a matter of taste - as long as he isn’t hurting anyone why should we care what he does in his private life.
As for bestiality, the Australian atheist philosopher Peter Singer has put together a good case for why bestiality should be legalized too - after all if homosexual relationships are just as natural and normal as traditional heterosexual relationships on what basis can you argue that other loving relationships are not?
Not if you ridicule them by showing how all their arguments are purely emotional and how they do not have a reasonable basis for allowing same sex marraige but not incestuous and polygamous marriage as well.
If they cannot respond satisfactorily to your arguments but still insist they are right, then they have no basis for believing what they do and are irrational - and not worth arguing with as they are behaving little better than animals.
You should not care so much what other people think - frankly most people are not very intelligent and prefer to go with the crowd than think for themselves- for me at least, all that matters is who is right and who is wrong.