Freedom From Religion
I have read the US Supreme Court decision in Lawrence v Texas, which led to the decision of the Massachusetts Supreme Court. I've carefully read the detailed argument against homosexuals [PDF] that one of those "family" organizations (United Families International) posts on their website. I also had to read up on incest law and polygamy, since those are part of the anti-gay arguments.
I think I'm beginning to get a handle on this issue. Most people emphasize the discrimination argument; that the refusal of the state to recognize same-sex marriages violates the Equal Protection Clause of the US Constitution. Laws that deny gays the right to marry are indeed discriminatory, but that's not the most powerful argument.
You sent me a quotation that sums up the central and most powerful argument for gay marriage:
"The legitimate powers of government extend to such acts only as are injurious to others."
In Lawrence v Texas the court told the Religious Right (Texas was their proxy) that the court would not enforce the religious views of the Religious Right. The Religious Right believes any sex besides marital coitus is sinful (hateful to God), but the court, in effect, ruled that sin and God have no place in a rational argument. Sin does not exist, so sin cannot be detected or measured (likewise God, of course). There is no way, therefore, to demonstrate that a given behavior is sinful. You may say it is sinful, another person may say it is not sinful, but it's a matter of personal opinion and there's no objective way to determine who is correct.
The court demanded the state demonstrate harm (injury). The Religious Right could not show that two men engaging in private, consensual, non-commercial sodomy injured anyone. The court said, our duty is not to enforce our version of morality; rather, our job is to delineate the limits of liberty.
So the court did not invoke the Equal Protection clause but rather invoked the Due Process clause. Religious concepts such as God's will and sin do not constitute a rational basis that justifies criminalizing or regulating private, consensual, non-commercial behavior. The state has limits to its power, and liberty allows people to do as they choose in private.
Well, the Religious Right freaked out because they knew exactly what that meant: the courts won't enforce religious beliefs. Liberty gives people the choice of whether to "sin" or not, in private at least. And sin does not by itself constitute harm.
Our government is of, by, and for the people. God has no place or power in our government. The real story here is separation of church and state, and Liberty.
The real reason the Religious Right opposes gay marriage is not because it threatens society but because it threatens their belief structure. Most people in this country believe, as I do, that people should be able to do what they want so long as they don't hurt anybody. Religious Right people think not hurting anybody is not so important as not offending God (not sinning). The problem with the Religious Right is that they want to force everybody else to obey (their interpretation of) God's will, even people like me who don't believe in God.
The Religious Right has tried to cast its arguments, which are really based on religious beliefs, as scientific or evidence-based, but the court saw through that. So now their only recourse is to change the US Constitution. If that fails, I suppose they can appeal to God.