i) One way of responding to SSM proponents is to press them on where they draw the line. For instance, if they approve of SSM, do they approve of pederasty? What about child prostitution?
Typically, SSM proponents stipulate certain criteria which make SSM acceptable, but pederasty unacceptable. Criteria like "love" and consent.
So, in their moral opinion, the only sexual relationships which the gov't should sanction are "loving relationships" between consenting adults.
But what makes their moral opinion truer or superior to the moral opinion of those who differ? The belief that marriage should be based on love is pretty ethnocentric. In many cultures, marriage is an essentially socioeconomic arrangement. Love is optional.
Likewise, prostitution isn't based on love. Hook-ups aren't based on love.
By the same token, mutual consent is pretty ethnocentric. Clearly ancient Greeks who practiced pederasty didn't consider that to be a necessary precondition. And it's not just a thing of the past. Pederasty is common in some Muslims societies.
The same holds true for gang-rape. Historically, that's commonplace when armies invade and conquer another country. Likewise, child prostitution is common in some Third World countries.
So what makes the social conditioning of SSM proponents morally superior to the social conditioning of pederasts? For that matter, what makes the social conditioning of SSM proponents morally superior to the social conditioning of their grandparents, who opposed SSM?
Are SSM proponents appealing to timeless, transcendent moral values? Is so, how are those grounded?
ii) When pressed, they might bite the bullet. They might admit that there are no objective moral norms.
But if they retreat into moral relativism or cultural relativism, then they can't simultaneous claim that homosexuals have a moral right to marriage. They can't insist that the state has an ethical obligation to sanction homosexual marriages.
So that's their dilemma. If, on the one hand, they draw lines between acceptable and unacceptable sexual relationships, they must explain what makes their moral opinions better or truer than those who differ. What's the secular basis for that?
If, on the other hand, they admit that these are ultimately arbitrary taboos, then they can't claim that homosexuals are morally entitled to marry.