What if he wanted to marry a girl aged 11? Or a donkey? Or a plastic life-sized doll?
your friendship is admirable, but we do not go with bells when a friend sins. We point out that they are doing something wrong. If it means not going for the act, it is obeying the wishes of God rather than man.
you can be nice about it -- 'I'm sorry I can't attend this due to my religious convictions, but I respect you as a friend.'
I think it cheapens the argument to have to inject bestiality or pedophilia or whatever the hell Latex-lovin' is called. If the problem is two adult men in love, then stick to that. Once you say it's against God's Word, it almost seems as if God's Word isn't enough all by Itself when you feel the need to invent slippery slope non-sequiturs.
under the law there is no “love test” for marriage.
it is never about love, it is about the institution benefiting society. Now we have judges and radical leftists pushing sexual recreation as grounds for marriage.
I think it's time to call in a specialist to comment.