If monogamic relationships were something that worked while both the guy and the girl do what they want and try to satisfy all their needs, there would have been no need to enforce monogamic relationships with laws (as it was for thousands of years, maybe even more).

The widespread existence of strong punishments to adultery and also how divorce was illegal everywhere is almost proof that without these rules relationships do not last. The collapse of the whole institution of marriage now that it is completely free and everyone does what they want is another big indication of this.

Everyone will end up alone now that there are no rules and we are all focused on short term well being. If we lose our beauty or our money, or if we stop offering any of the “objective benefits” we used to give them, our relationship is in danger.

Sexual freedom and “freedom of love” is incompatible with stability of relationships. Im pretty sure this freedom has been tried many times before by hundreds of civilizations, but it always ends the same way: it fails.

Of course nobody needs marriage. At least not when they are still desireable.

But being 60+ alone is probably bad enough that its worth it to sacrifice some freedom while we are 40 in order to get there (nobody starts loving us when we are 60+, or at least it is extremely rare)