DY- What I mean is that it’s not the government’s job to enforce morality (at least in a government that’s not a theocracy). The point of government is, as you said, pragmatic. To make sure that society functions and there is no anarchy. Morality belongs to the rabbis, imams, pastors, etc…
We know from the Torah that gay marriage is destructive to society on a physical level. Chazal talk about gay marriage in reference to the mabul. But from those who don’t accept our Divrei Chazal as authoritative, then gay marriage is purely a moral issue. It should be legal as the government should not be legislating morality.