As RR posted, studies so far suggest that children of same sex parents are doing just fine. Secondly, much like heterosexual couples, having children doesn't legitimise anything. That's as offensive as comparing same sex couples and their relationships to mere friendships and there's nothing legit about a same sex relationship.
TBH, I'm fairly close minded when it comes to understanding bigots. I just can't bring myself to do it, especially when the religion card is played. Is perhaps somewhat ironic but it's how I am.