I don't necessarily disagree with your point, and I certainly do want to find the strongest way to convince the necessary one or two "swing" votes on the Supreme Court not to overturn state laws requiring that marriage be between one man and one woman.
We don't need arguments that will convince Justice Scalia and others who will already agree with us, even though in the long run those are the only arguments which aren't based on shifting sand. In the current political and judicial crisis, we need arguments that will convince people who have already compromised on key principles of traditional values, but may not want to compromise on this issue.
So I may be in agreement.
But here's the problem. In a culture in which lots of people claim the world is overpopulated and in which lots of people advocate reducing population growth, not increasing it, how do we claim that "society has all incentive to encourage production of children" without basing that claim on religious principles?
And if we do that, we're back to the same problem of dealing with justices who reject religion and claim that the objections to same-sex "marriage" have no rational basis in secular (i.e., non-Judeo-Christian) principles.
I'm not sure how we get around this with legal logic. I'm afraid nothing will work apart from raw political power. And I am increasingly concerned that public opinion is moving the wrong way.
less children less taxpayers.