Very good article here:
What a healthy marriage culture does is encourage adults to arrange their lives so that as many children as possible are raised and nurtured by their biological parents in a common household.
We cannot say with any confidence that legal recognition of same-sex marriage would cause infidelity or illegitimacy to increase; we can say that it would make the countervailing norms, and the public policy of marriage itself, incoherent. The symbolic message of inclusion for same-sex couples — in an institution that makes no sense for them — would be coupled with another message: that marriage is about the desires of adults rather than the interests of children.
The article is of course written from an entirely non-Biblical perspective, but I think it has some arguments from a philosophical and practical standpoint that Bible believers can use.