Earlier quoted context omitted.
> Homosexual couples do adopt children and given the number of children in the foster care [...] A commonly cited argument, to be sure. Let's look at the magnitude of its impact in practice. Per federal statistics, roughly five hundred thousand US children are in foster care, which is roughly two-thirds of a percent of children in the US, or six per thousand. Figures on the number of children adopted by homosexual co…
As I replied to your linked comment, you're conflating the arguments. If you believe that marriage should be allowed if and only if a couple bears/adopts a child, then make parenthood a prerequsite for marriage, there's no point in instituting complex heuristics to try to guess whether a couple wants kids or not beforehand.
The essential difference I see isn't one of individual probability of parenthood, but of aggregate. Not all heterosexual couples rear children, true. But the likelihood of a randomly selected example rearing children is vastly higher than that for a randomly selected homosexual couple. That, I argue, is what makes it worthwhile for the state to support the former and not the latter.
To go into individual personal detail, as you seem to think I advocate, might in theory be a natural corollary of what I describe - but to do so in practice not only massively increases the bureaucracy and cost involved and brings in creepy edge cases, but requires a panopticon state in order to work.