Take my paternal grandparents. When my grandmother was still alive, the two of them happily lived together and cared for one another even though my grandfather was at that point experiencing the early onset of dementia.
When she died, my grandfather had to go into residential care, paid for partly by their pension, partly by my parents and uncles, and partly through the subsidy that the state provide for elderly people going into care homes.
The more people who are able to live together in a mutually caring relationship like a long-term marriage, the less people need to go into care homes meaning the less subsidy needs to go into that kind of care for the elderly.
Marriage has a societal benefit in promoting that kind of mutual care, regardless of the gender of the partners and regardless of whether you have kids (and, hell, same-sex couples can and do have kids, so that's a shit argument anyway).
Could you set up another system to provide that kind of benefit? Sure. But this is the system we have and it should be available to long-term gay couples as it is for straight couples.
Disclaimer: I'm single, so I'm not arguing for these benefits. That's just what I always thought the reason was.
Married couples in the US most get tax benefits to the extent to which they have a significant imbalance in income, and it seems to be a lot more connected to the idea of traditional division of labor (one partner working in the trade economy while the other provides in-home support for that partner) than children, per se.
They are tax benefits that directly subsidize children, but they aren't conditioned on marriage.
> I find it weird that this argument has so far been completely ignored by either side in the pro/anti-gay marriage discussion (at least as far as I've seen).
The idea that the civil benefits of marriage are inherently tied to the purpose of promoting reproduction are rather central arguments in the anti-same-sex-marriage camp; this was, in fact, the main argument for Prop. 8's constitutionality offered by its proponents.
My apologies then, I haven't followed the gay marriage discussion in general, only the discussion about Brendan Eich.
The injustice is simply in the sit-in-the-back-of-the-bus thing - that a lesbian couple should be stamped 'civil union' just so everybody knows they're not as good. Its Jim Crow all over again.