That's an interesting perspective. I disagree with many particulars, but I'm not sure they matter.
"A monogamous relationship is a monogamous relationship." True, that. And irrelevant (imho) to legal marriage. Legal marriage is what forces third parties to treat me and my wife in a special way. So, for example, her employer-provided medical insurance covers me, but would not cover her (hypothetical) ten boyfriends. Also, she is on an international assignment, and they limit some of the benefits provided based on family size. I count, but her niece and nephew don't. Since we don't have kids, we don't get the awesome free tuition to an international school near us, and (importantly) we don't get to just pick some random relatives (or even our niece and nephew) to get it instead.
Even my Sam's Club membership automatically includes her, at no extra cost. They leverage the institution of marriage as a way of limiting the scope of the benefit of two people using one membership in a way that mostly works well for the people who act on each other's behalf.
Also, when those monogamous relationships fall apart, there is often disagreement on how commonly held property and (previously) shared responsibilities are split. That can require some form of mediation, especially because there is often a large power/status difference between the parties. That fact does not change if the government gets out of the business of pre authorizing the relationships that get special legal mediation.
So, I know you must be tired of people trying to convince you of the social benefit of legal marriage, but I'm going to try one more tactic. So many people get married and get a benefit from it, both personally (which you dismiss) and socially (which you seem to ignore) that I hope you try to see this from their perspective. Legal marriage will endure whether you join it or not (I hope). Please don't be against it; just don't join it.
Next topic, a particular that I think does matter: Taxes! "I pay taxes so their little snot-nosed kids can go to school, and they get a tax break? Why isn't there a kid tax?"
I don't know why you pay taxes - you could always move to Somalia. But I'll tell you why I feel comfortable with my government forcing you to pay taxes, especially for education. The education is not just for the benefit of the child or parent. That education benefits society. An educated populace can invent new great things for us all to use. An educated populace (in a democracy) is critical for good collective decision making. An educated populace is wealthier and safer to be in. Because you benefit from those things, my government takes a bite out of your wealth, even though you are short-sighted, tight, and stupid, as evidenced by your desire not to support those things. I wish your education had been better, but I can only imagine the horror of your ideas if you hadn't gotten at least as much as you did.