I'm all for monogamy (tried it the other way; doesn't really work for me). It's in more the marriage part that I'm lost. I get the IRL argument for it (there are legal benefits and all) but does the ceremony itself really matter to people in characterising their relationship? Again, genuinely asking.
Old question, but I thought I'd go ahead and answer.
I used to ask that same question because it seemed like nothing but a legal construction, but as I get older my opinion has changed somewhat. It's specifically because marriage *is* a legal construction (one that is sort of hard to get out of) that makes the difference. Sure, people get married without really understanding what marriage means and then regret doing so, but I think there's still a level of separation in people who get married and people who don't.
People who understand what marriage is, truly understand, and do it anyway are literally saying "I want to be with this person so much that I am willing to give them various rights to my possessions, what happens to my body, and my children, and I want the state (and society) to regard me as no longer being an individual but part of a singular unit made up of two people." Some people in long term monogamous relationships behave this way, which makes them functionally married, I suppose. But in my experience, when long term monogamous couples are asked why they *don't* marry the answer is more or less "divorce is messy" which means they are at least entertaining the notion that they aren't 100% committed to their partner until death and thus aren't willing to gambol with the consequences of breaking up a marriage.
Does marriage live up to the ideal I've presented all the time? No. But that's the function that it's supposed to serve, and people who go into it are supposed to understand it's for keeps, which is why the consequences or getting out of it are onerous. People who avoid it (generally wisely in their individual cases) usually do so because they understand it's for keeps. The ceremony serves as a public declaration of a person's intent to accept the risks of marriage. It might not be necessary, but it is definitely taking the relationship up to a new level of legal and societal commitment.