This same sex marriage thing is a no-brainer.
There are social and religious issues surrounding this subject but the only one we can decide with any stability is legal issues. It really is a legal issue, not a religious one, and as a legal issue there can be only one answer--to make homosexual and heterosexual marriages legally equal.
In our culture we define marriage as a mutually agreed on union of two people and only two people. No outside person can intrude into this union so my marriage is different from every other marriage as they are different from each other. Each one is a unique and isolated case with no tangible impact on any other marriage.
It would be different if homosexuals wanted to horn into other marriages or force people to enter same-sex marriages but they don't. They want to form their own mutually agreed upon marriages, completely separate from every other marriage.
The question of whether homosexuality is a sin is moot in this argument. In our country sin and law are completely separate as described by the the constitution. For instance: there is no law regarding keeping the sabbath or worshiping idols, both of which are listed in the ten commandments. If we were going to incorporate sin into law you'd think these would be first on the list.
We can and do allow religious groups to make their own determinations about marriage independent of the law. For instance: catholics don't recognize legal divorces. If a divorced person gets remarried but doesn't have their first marriage annulled, then the Catholic church doesn't recognize their new marriage, but it has no impact on the legal status of the marriage.
This could be a model for same-sex marriages. Some religious groups would recognize them and some wouldn't based on their own interpretation, but before the law they would be the same as all other marriages.
There is also the constitutional issue of equal protection to consider. If the state doesn't recognize same-sex marriages then how can they say they offer equal protection to homosexuals?
Trying to decide issues of law based on the concept of sin is a very slippery slope and one our founders provided us an escape from by separating church and state.
There are social repercussions to this legal issue but the social concept of marriage and family were experiencing huge changes long before we brought the issue of homosexuality into it.
We passed the point of no return a long time ago when circumstances made it possible for women to survive without marriage. Marriage used to be a matter of survival, now it's a matter of choice and the only question is how we define it in the future, and like-it-or-not, that future includes the social, and more importantly, legal rights of homosexuals.