If I may offer my take:
I don't think the mere fact that something is sinful makes it unlawful (at least not in our legal system).
I also don't think that people (gay or straight) have an automatic birthright to have legally recognized marital unions.
In fact, I don't think the government has any business deciding which benefits any cohabiting couple can give or withhold from each other.
The typical argument is "Gays should be allowed the same rights as straights," by which most usually mean "we want government cheese, too."
I deny that straight couples should be allowed even those rights to begin with, at least not insofar as those rights are state-sponsored / -determined.
I think all people everywhere should be allowed to enter into voluntary contracts for whatever benefits they agree upon mutually, and the state stay out of it.
This would mean that all couples, straight or gay, married or strangers, would be allowed to declare beneficiaries, power of attorney, tax co-sharing, etc however they please.
It would also mean that all couples would be able to mutually agree not to exercise any of these rights, and could opt out of any state-recognized unions.
I hate that my marriage has to be "validated" by the state. I'm not sure why they have any business telling me whether I am or am not married.
I don't understand why gays or straights want to grant the government any more power to control how or with whom we live.
I don't think the government has any right telling people who to give legal or financial benefits to on the basis of a romantic covenant.
"By the power vested in me by the state of *********" should not be a phrase that exists in our vocabulary. It's sheer insanity.
Some say "Keep the government out of marriage, and allow whoever to get a civil union..."
But I say "Keep the government out. Period."
The government subsidizes things because it thinks they are good for the country at large (or, at least, good for a part of the country).
The subsidy on marriage (added benefits, lower taxes, etc, etc, etc ad nauseum) is, in my view, because marriage is good for society.
The government doesn't get involved in marriage because it cares what's good for those two particular people. It does so for the public good.
The public good that is perceived is stable homes, financially secure child-raising, and time-tested benefits like lower aggression / violence.
The question of gay rights is backwards. Governments are aggregates. They don't (and can't) care about individuals, regardless of what anyone says.
The real issue is whether gay "marriage" is good for society in the same way or a different way than traditional marriage. If the same, then equal rights.
If gay marriage (because of procreation, higher separation rates, more violence, whatever) is good for society in a different way ("civil rights"), then different rights should exist.
I'm not sure that much has been proven conclusively on either side, but studies do exist. Is my marriage good for society? Is a gay person's relationship? I don't know, politically.