Without children, marriage is ephemeral.
The need to formalise a relationship with a marriage ceremony is traditionally linked to religion.
Without religion, children or a stable income, why would a person need or want to get married?
When I was in my late 20s, I also had good chemistry with a female friend who was 32 or 33. She hinted pretty strongly on several occasions that she thought we'd make a good couple. Unfortunately, she had a poor relationship with her father which is a no-go for me due to past bad experiences. I remember she confided in me that she felt like she needed to start looking to date guys in their late 40s/early 50s. She was good looking and a graduate student at Columbia University at the time. A lot of women in their mid 30s who should have very good dating prospects have a lot of stress about being passed over for younger women, and unfortunately they're often not wrong to worry.
If you can't imagine leaving your partner for someone else, and they're worried about being left because they're seeing their friends getting left, words are cheap and your partner will know it. If you can't imagine leaving your partner, then there aren't many downsides to getting married, and your partner will also know this.
Of course, there are others in different situations, but that's the experience of me, my wife, some of my wife's single female friends, and some of my single female friends in their mid-30s. Several of them very pleasant good looking women with graduate degrees from Ivy League schools and good careers.
So, for me, I had gotten a bit tired of the dating scene, really liked my partner, couldn't imagine breaking up, and knew my parter was getting anxious. For us, it was a good move, and has made our relationship much stronger.
Genuinely curious why this was (for you) a leading indicator of a bad relationship?
I've had one girlfriend who was essentially abandoned by her father and another whose father was essentially indifferent to her emotional well being or success in life. It was pretty obvious that at least for both of them, they always had a nagging feeling that I seemed to love and support them, but maybe it was just a charade. After noticing the connection, I actively sought out women with supportive fathers who clearly cared for their daughters.
My father-in-law is strict, but unquestionably supportive of my wife. My wife talks to her dad on the phone at least 5 times per week. When we were first dating, I consciously asked my wife about her father and watched her body language and listened to the way she talked about him.
Even early in our dating, my wife was far more likely to have a charitable interpretation of my actions/motivations vs. my ex-girlfriends whose fathers showed little evidence of caring. On the other hand, my wife still calls me 2-3 times per day at work, but it's a small price to pay for her subconsciously believing I genuinely have an altruistic interest in her well being.
Edit: there's also a heavy dose of cultural and personal context here. Her calling isn't being needy, but feeling culturally duty-bound. In her home country, as the only daughter, it's considered her responsibility to take care of her parents. Her brother lives 5 minutes from the parents, and he makes sure they're taken care of, but my wife feels a bit guilty living in another country and wants to make sure her parents don't feel abandoned. One of her father's sisters married an American, moved to the U.S., and slowly lost touch with the rest of the family, so she wants to ease any anxiety her parents have around that. Also, as much as I remind her that it's not her job to cook, as soon as she finishes lunch (at her place of work), she feels like a bad wife if she doesn't consult with me about what she should cook for dinner and what time she should start cooking to have dinner perfectly timed for my return from work. I've tried a bit to get her to stop asking what I want for dinner, but you can't change a person, and it is very sweet, other than it makes me feel a bit awkward. She cooks and I clean, and she finds cooking a good way to de-stress after work, but I still feel a bit bad that she feels like it's her duty to cook. She also talks with her brother's kids one or two times a week to help get them practice with their English. She's really trying to help out her whole family and keep everyone connected across the miles. Thailand: land of spicy food and sweet people.
I wonder if there is any standard place I can put the information on what to do if I am on brink of death somewhere so hospitals know to check for it.
They were applying for jobs in the same place, same people interviewing them etc. People didnt know they were dating.
When they found out he was partnered, they treated it very seriously, as if they were in a committed relationship, deserving of protection etc. When they found out she was partnered, it was different, he was just a boyfriend of who knows what status etc.
They decided to get married after that experience.
As others have noted, the purpose of marriage is to protect the relationship, but what that really means is protecting each other. In some cases this means ensuring equity of treatment.
Similar cases have shown up with COVID. Every couple of months this year I've listened to news stories about couples dealing with cross border restrictions on being able to see each other. The last one included a couple who had been together for years and were despairing about the fact that protections of border crossing only extended to married couples. I'm not suggesting judgement should be cast on them for not being married, as that's between them, but it's another example of how representing the significance of the relationship to the state can have consequences.
For some, publicly declaring your commitment to one another can have a powerful affect on the relationship. It can communicate a shared vision and commitment for a lifetime. It is not requisite for that commitment, but it leverages cultural support structures that can help keep a relationship healthy.
For me personally, I believe marriage provides a couple with an important concept that they are part of something greater than themselves. This is not an exclusively religious idea (I consider myself an atheist). When times are difficult (and they will be over the course of a lifetime), this bond can help a relationship survive when otherwise the pressure to give up can be significant.
It is that way because it was the default for a long time. Now it is shifting and it will take time for society to adjust to this new reality.
Look into the marriage tax penalty, in some situations you will pay more (both high income earners)
I suppose there are legal reasons to get married.
I don't think most people get married for tax reasons.