> Your entire post is based on the premise that each culture's practices are all valid within the culture that they are practised within.
Not really. I will explain below.
> This is demonstrably false.
While I would not necessarily agree with the validity view you said, I don't think it is demonstrably false. Please demonstrate that it is false in a way unbiased by your own cultural assumptions of autonomy.
> The fact that Russians support each other via a traditional heterosexual family unit does not in any way mean that someone who is gay should be discriminated against.
Define discriminated against. Is a retire-with-the-kids culture where parents get to say no to spousal choices (but won't if he's made her pregnant) inherently discriminatory?
> The whole premise of your argument is that gays cannot retire with children, so they will be a burden upon the state.
No. My point was that same-sex couples can't have equality if only opposite-sex couples get a veto override (via "accidental" pregnancy) against parental objections. If equality is the goal, one cannot tolerate a retire with the kids culture because parents have so much power over childrens' choice of marriage partners, and this is inherently greater with regard to same-sex couples.
I don't think you read what I wrote very carefully.
As for the cultural validity piece, as I said, I would come back to it. I don't think it can be demonstrated to be false, because cultures function in different ways to the point where apples to apples comparisons are essentially impossible, and so any comparison comes from assumptions, passed on by culture, which determine the conclusion. But at the same time there are certain problems that can't be easily addressed by a position of pure cultural relativity. In my view, these come in two forms: inter-group disputes and breakdown of institutions.
The first includes things like intercontinental slave trade, Germany gassing Polish Jews, Japan's brutal treatment of various occupied areas in WWII, colonial treatment of Africa and Asia, treatment of Native Americans by the US government, etc. You can't solve an intergroup dispute by assuming that both sides are valid actors, so what one is left with is a political resolution.
The second issue is the question of the breakdown of institutions. If a practice is perfectly valid in a culture, then institutions should be perfect, but they aren't anywhere. I think that critique of cultural institutions from a context sympathetic to the culture is a worthwhile endeavor.