female partners are typically notoriously high maintenance in money, attention, and emotional labor.
That’s the stereotype, but men are the ones who die sooner if divorced, which suggests they’re getting a lot out of marriage.
ETA: looked it up, divorced women die sooner as well, but the effect is smaller despite divorce having a bigger financial impact on women.
Assuming arguendo this is true: if you care primarily about sex, hiring sex workers is orders of magnitude more efficient than marriage. Therefor the existence of a given marriage is evidence both sides get something out of it besides sex.