My mother-in-law was married to this one guy for a decade or so. Then divorced, then they moved back together, split up again, ...
Anyway, despite the paperwork, they were a couple. Their emotions and finances and possessions and relationships were all tangled up on a big knot. My kids called him Grandpa. They were closer to each other (even when they were living apart) than either was to anybody else.
So one day the guy croaks semi-unexpectedly. And before he's cold - I mean that in the most literal sense - his relatives who haven't talked to him in years are pushing her out of the funeral arangements and carting off "his" stuff, a substantial part of which she's still making payments on.
She's a tough old bird and mostly came out OK. But she had no rights at all, because of that divorce paper. She had to rely on her rather ummm robust personality.
That conflict could very well have ended up involving civil authorities.
I've heard from others that one partner has been kept from the hospital bed of another, either by relatives or by hospital rules.
What marriage means is official recognition that two people are bound together by emotions, relationships, and posessions, and that they have a relationship that outranks all other relationships. Not just in sentimental terms, but in terms of property and mutual decision-making.
If one of a couple is incapacitated, without marriage the other person has no legal right to express the incapacitated one's wishes. Any relative at all, regardless of how well that relative knew the incapacitated, has the absolute right to overrule the partner's decisions. And that partner almost certainly knows what the incapacitated one would decide better than anyone else.
So if you are gay, whether your organs get donated, when the plug gets pulled, and all the other stuff, gets decided by someone other than the one who knows you best. Someone who gets to inherit your stuff and probably half of your partner's stuff as well. And if the relative is in a pissy mood, your partner won't be by your side at the end.
I'd like to see legal recognition of life-partnerships in general. There are quite a few people who live together as partners without, presumably, any sexual dimension. You don't think every pair of elderly sisters living together are incestuous lesbians, do you? Their partnership deserves protection. Granted, if they are actual sisters their is some next-of-kin recognition, but they often aren't. I know several of those pairs who used to be nuns - their sisterhood is quite real, but not next-of-kin.
Marriage is a really odd kind of corporation. The only one where sex is a legal part of the deal. The only one where the corporation owns ALL of your assets and has an enforceable claim on all future assets.