[user234683]

This is a good time to consult Chesterton's fence [1]. If you go up to something that's existed for thousands of years, and find it tempting to declare it pointless cruft that should be swept away in the march of progress, maybe there's something you're missing. I don't have time to make an extended argument but one idea is that the most important part of marriage isn't for the happiness/benefit/pleasure/romance of the couples, but for the benefit of children. Studies demonstrate that children of single-mothers have worse outcomes, such as worse mental health outcomes. A marriage is supposed to be difficult to get out of, to disincentivize broken families. Not that this is anywhere close to a full answer; cultural evolution [2] means we may never get to a full understanding of whether/why the customs we have are beneficial, simply because societal dynamics are extremely complicated and still beyond our understanding.

[1] https://fs.blog/2020/03/chestertons-fence/

[2] https://slatestarcodex.com/2019/06/04/book-review-the-secret...