A few suggestions of mine. No-one is expected to fully (or, for that matter, even partly) agree. Hope I won't shock anybody.
I think we should not put everything in the same "bag", but distinguish between situations. Our sex life is determined by diverse urges and conflicting priorities. Also, "nature" and "culture" probably often clash in such matters.
Sex. "Nature". Sex is one of the most deeply ingrained instincts in the human psyche, along with self-preservation. I can't think why women wouldn't share in it equally with men. I think a healthy single woman, in the contraceptive pill era, can be just as tempted as a man by an overnight no-strings-attached affair with a good "performer". What may hold her back is "culture" - centuries of religious/moral coercion mostly imposed by men (see below) - and a concern for her social image.
As for men, I think sex has always somehow been connected with "power". Men are caught in a sort of tacit Darwinian competition for more… - more land, more money, more glory, more political power... and more women. Collecting women is part of that quest for fame, for influence. It comes down to saying : "See, guys… I'm the best". It's that "mine is bigger" kind of stuff. It all begins in High School, and you get trapped in it, whether you like it or not.
Procreation. Also a deeply rooted "need", linked to that notion that having children is somehow a way of "surviving oneself". In this perspective, after millenia of a perilous life in often precarious conditions, women may have learnt to pick their men on one criterion : reliability. Then they would no longer look for a sex mate, but for a father, who would help, care and protect. I'll concede it's no longer true in modern industrialized societies, where women can cope alone, and sometimes choose to. But millenia of fear probably left a lasting subconscious trace. In a wilder world, men, being on the whole more muscular, more aggressive, not subjected to the hazards of pregnancies, were more efficient in fighting off predators and enemies, providing, etc... (Hence patriarchy, probably). Before the pill, with risks of pregnancy running high, and knowing what men are like, it was no doubt safer for a woman to put her partner's commitment to the test beforehand, just in case…
On the men's side, there was always that lingering fear that the child might not be his own. To remove the doubts, men used both brawn and brain, chose to constrain and compel, and invented various tricks : social discredit, religious proscriptions, harems... Faithfulness in their women guaranteed better chances of legitimate offspring.
So priorities were different, but pointed towards the same end : institutional monogamy.
Marriage. Logically followed. But marriage is a long-term contract. So it involves other forms of attachment than just mutual physical attraction or a "paternal profile" : respect, tenderness, shared values, etc… basically, what we call "love". Marriage, in my opinion, is not "natural". It is "cultural". It helps sublimate desires that would otherwise be among the most animal ones in humans. For centuries, women were married off to men they had hardly met. Not all those marriages were failures. When people prove considerate, things can work out. And sex, whether satisfactory or not, becomes, to a degree, a side issue.
I think we are all heirs to those conflicting temptations and injunctions. Then upbringing, moral standards, experience – and maybe mere chance – make the difference.