@FlyingOink
Historically in the west reproductive role has been one of the most basic purposes of marriage, more so than romantic love by a fair way. So it was an entirely legitimate discussion to have, whether it made sense or was socially useful to have an institution that was predicated and revolved around issues of sexual reproduction, whether that purpose was in any way relevant in modern society, or if it was still useful, whether admitting same sex couples would undermine in some way the basic principle in the same way that saying women could include a person with the male reproductive role could end up undermining women's rights legislation.
That's a fair point, uncomfortable for me to think about but worth discussing.
The premise that marriage is about reproduction is why annulments can be granted for reasons of infertility or impotence. However if marriage was only about reproduction, then post menopausal women would not be permitted to marry. I don't know any religion that prohibits this, in some cases widows are supposed to be married off to their brother in law, (and there is no mention of her age) for example.
So there is an understanding that some marriages may not produce children, but that they are a family unit in themselves and that the spouse is next of kin, has power of attorney, etc. Those rights should be shared by same sex couples, as the institution of marriage (or indeed Civil Partnership) is neither religious nor dependent on fecundity.
If we didn't have civil marriages, in registry offices or hotels or wherever, and marriage was entirely a matter for a religious organisation and concerned the production of offspring within a stable family unit only, I'm sure that taxes etc couldn't be affected by marriage status as that would be preferential treatment for religious people, and also there would be zero provision for non-believer straight people to have any form of marriage.
The reasons for allowing people who were unlikely to have children is a little less straightforward than we tend to assume I think.Part of it was simply practical - for people who suffered from infertility it wasn't typically clear why that was or whether it was likely to be permanent, it may have been completely unknown before they were married.
But there are other factors too - one being that in many cases where people were known to be incapable of producing children, it was considered to be a better thing if they did not in fact marry. It was also not uncommon and was considered to be completely reasonable for people past childbearing age to live apart or enter monastic life if their social position allowed it.
But the Christian European view of illness, and that includes age, was that potentially it was something that could be healed. If not in fact, who knows, a miracle might even happen. There were after all examples of very old mothers in the Old Testament, and in Christian belief in the New Jerusalem everyone would be healed of their afflictions.
Widows being married to brothers is interesting because it's in part about social care, which arguably we manage other ways. But we could maybe compare it to things like pension arrangements and such which relate to marriage but not procreation. Although the other reason that widows were sometimes married to their brother-in-law was so they could produce a child to inherit the first husband's property. The brother in law effectively provides an heir for his brother.
I tend to think that if marriage was a personal matter only, you would either find non-religious people would simply forgo it, seeing it as irrelevant (which many do now) or they would set up their own types of observances, like some of the humanist groups do. There are after all plenty of completely secular rituals that we have as a culture.
But I do think there is a really significant difference between discussing the pros and cons of an institutionally defined partnership that is fundamentally predicated on potential reproductive capacity, and the way it's been approached in a number of countries as a matter of rights - essentially saying that a nation has no right or mandate to create an institution or laws based around biological sex categories - that to do so is fundamentally discriminatory. I think that sets up a really dangerous precedent that could, and maybe has, had significant implications in other areas. And also in the minds of citizens making decisions about the direction of our culture.