I suppose the first thing that comes to mind are the horror stories I’ve heard about what can happen when a loved one is hospitalized and you’re not their legal spouse. Your loved ones’ estranged parents might be making medical decisions on their behalf and you could be entirely cut off from visiting or even know what’s going on health wise with them. There are the other legal benefits that come with marriage including tax breaks, inheritance, and community assets. You can set up some of this with things like power of attorney and a solid will, but why not just get married instead?
Speaking as a gay person, we had and have each other’s powers of attorney for those fun times when the hospital doesn’t acknowledge our marriage or the rights that come with it. This is to say that there are ways to set this up. I hold certain powers for a married family member with no children, though a different family member would otherwise be next in the decision-making sequence. Even though we’re married, my wife and I have legally identified survivorship status. Our wills are explicit, our advance directives are very explicit.
I have friends on disability (like, parapalegic) who didn’t marry because they’d lose their benefits.
>>tax breaks Tax breaks for unmarried people, or no incentives for married people, would be sporting.
That’s why people do get married. I have no problem understanding that very basic concept. I was explaining why they don’t which is equally simplistic even if it’s not your preference.
Its why my ex and I didnt marry. He thought he’d lose his s.s.
I suppose the reason people maintain a “long-term, long-distance, low-commitment, casual girlfriend” style relationship for years is that you can basically up and leave at any time. The risks of being contractually obligated to stay with someone while intermingling finances and whatnot would seem to outweigh the benefits of having someone to prevent doctors from pulling the plug on you or signing for your personal effects in the distant future.
The marriages I watched implode were usually those where both couples seem to have wildly disparate personality types, goals, ambitions, or values, often they come from very different social classes, or he married a stripper, or she married the sort of man who marries a stripper, or he’s just generally a douchey guy, or one (but not both) of them are fucking out of their minds.
I lot of these marriages I had pegged at the wedding.
What’s a lot harder to predict IMHO is how partners might behave differently under stressful situations or changing life circumstances.
I’ve wondered… are you both roughly the same skill level? What’s gonna happen when one of you starts crushing the other on the boards??? 
Keep in mind that this all happened in the 1980s. Anyway, one of my college friends had been divorced (no kids) and also lived with a guy (also no kids) but she and the lived-with guy bought a house together. The divorce had not been amicable, but the split with the second guy was, and she said that getting uncoupled from that house was worse than her divorce.
My parents were from a generation where it was a given that if a girl had a boyfriend when she graduated from high school, that was probably who she married, even if they really didn’t want to. This kind of thing lasted, to some degree, into the 1970s, and a lot of disastrous mistakes were made, that’s for sure.
The one man I thought I would marry? The physical attraction just wasn’t there, and no, you can’t force it either, and we ended up not having as much in common as we initially thought we did. We didn’t stay friends, but we were friendLY and that’s always a good thing. One thing that would have been a very bad sticking point had we married was that I did not want children (and didn’t have them) but he did. Were I to have wanted children, he definitely would have been the kind of man I would want to co-parent with, but I didn’t want to be a mom.
I saw on Facebook that he married someone else a few years later and they had two kids, and then split up when the kids were teenagers, about 10 years ago. It must have been one of the most amicable divorces on earth (among other things, they never unfriended each other on Facebook, and lived about 2 blocks apart) and when he announced he was going to remarry, the first person to offer congratulations, complete with an e-confetti cannon was, you guessed it, his ex-wife.
Around twenty years ago, one of my friends went through a divorce with his wife in Santa Barbara. They owned a house that massively appreciated and they had two young pre-teens. There was no way they could afford two houses in SB so they moved to a different city together and bought houses an easy walking distance apart so they could co-parent more easily. It was a great plan.
Close enough to be competitive. I taught her how to play chess. It took her ~100 games before she beat me. She was fine with that, she’s very competitive being a former IronMan and all.
We’ve played thousands of games of chess. Not one has been like another.
We also play Cribbage. Her work background envolves numbers so she is a bit better than I. My background is GIS so being very spatially orientated, I’m better at chess.
Darts is a toss up 
As a divorced man, who literally lost his house during the divorce, I do still support marriage.
I did not make those vows “in fitness, in health, for richer or poorer” etc.
But I lost my dad to cancer at age 8, and the repercussions of that have haunted me all my life. My divorce, when our children were a similar age devastated me.
I am kind of old school, I even asked my ex-wife’s father for permission (he, too, is quite old school).
But yet… marriage is like trying out for the 1st team and getting in. The eventual family unit is quite incredible.
I tend towards long term relationships- my ex and I were together 11 years, my previous g/f and I for 9 years.
Marriage is just a tradional social construct, but one I take reasonably seriously. I doubt I will ever marry again - but that is based on the fact that it was the most emotionally damaging breakup I have ever experienced.
I also tend toward long-term relationships. I got married for the first time a couple years ago, but we’ve been together for a decade already. I don’t feel much if any different now that we’re married, but I still have a hunch a divorce would be much worse than my other breakups.
There may be indicators, however, if you know where to look. A guy once told me that the best way to see how your love interest is going to look and behave down the road is to observe her mother.
I see my community is offering a class for young people about how to identify potentially abusive partners when dating. I think that’s wonderful! I know if I’d had such a course it would have been helpful. No doubt I’d still have made dumb choices, but I think I’d have realized sooner what was happening and what to do about it.
Gawd.
Mrs. J.'s mother, while always nice to me, was miles from her daughter in looks and attitude. If I’d thought that her daughter would turn out similarly, I would’ve run fast and long in the opposite direction.
That’s a really old canard and mostly untrue. I wonder why it’s always about mothers and daughters, but not about fathers and sons.
My mother is very, very different to me, although my sister is quite similar to her really intelligent, driven, etc. She (my sister) has been happily married for, I don’t know, 15 years now.
My ex-wife’s mother - aside from them both being very creative - is also a very insecure woman, whereas my ex wife is very emotionally secure.
I think the premise is a fallacy.
A lot of them are. LOL
Cf. the scene in Chad Kultgen’s The Average American Male when the protagonist meets his new girlfriend’s parents: the mother reminds him of an older version of the girlfriend, and the father reminds him of his last girlfriend’s father.
I mean, other posters have given you ample perspective on the other points you brought up, but this statement is just flat out wrong. Many, many contracts are at will, auto-renewing, or indefinite but cancelable at any time. I’m not sure where you got the idea that all contracts are time defined.
I’d mostly heard it about how a young woman will look as she ages.
Regardless, in my anecdotal experience, if it was about behavior, it wasn’t accurate in our case; my wife’s behavior (and, frankly, her looks, too) comes from her father. Her angry, crazy sister, on the other hand, does probably get a lot of that from their mother.