On marriage, sexual liberation, prostitution and natural selection

If there are equal amount of men and women, the women will only sleep with the best of the men. All the women will get laid, but some of the men don't. Leading to male scarcity: the men who don't get laid will not contribute to that society, because they do not feel wanted. Those men won't do anything for people who have nothing to give in return, because no one works for free; "I'll give you stuff, if you give me stuff" - and this gives birth to prostitution.

At first the going rate is high overall, but then the most attractive women will get all the customers, then the less attractive women lower their prices, sparking competition (because otherwise one might overprice themselves out the market), eventually driving the prices so low that exclusive deals appear more lucrative and there we have marriage.

If the prostitutes try to game the system by agreeing on a certain price point, the best value-for-money-women will still have the customers, and some of the men won't value sex that much, thus inventing alternatives for their fulfillment. The women who are left without customers, either:

1. Decide not to adhere to the agreed price point, starting the competition. Leading to exclusive deals = Marriage
2. Want a share of the profits made by the more preferred prostitutes (e.g. having union membership fees, which is essentially communism), this will lead to the prostitutes hiding their income, making money on the side. Starting the competition, leading eventually exclusive deals = Marriage
3. Choose another line of work (for example cleaning a brothel). Which is to do other people's things to have a roof over your head and to have something to eat, which is essentially about the same as being a housewife.

You either end up supporting the best (doing the work they can't be bothered with) or competing with them. In either case, you'll live in their shadow, unless you are the best.

tl;dr: All things being equal the best will win. (No matter what rules you have, the best within those rules will win.)

It's natural selection: Smart and good looking people win. People who have good genes, look good to us (that's what being good looking means: having good genes.) Ever noticed how people shower good looking people with money, affection and support (cosplayers, instagram models, titty-streamers, cam..people..): They want a piece of that action. And the ones who have the most wealth/influence, get the attention of the good looking, thus combining the genetically best and intellectually brightest. Intellect allows you to generate more wealth with less effort, you are more influential with what you have got.
The more you have the "looks" and "smarts", the more you win.

PS. Also there's the thing if you are very intelligent, but have the shittiest genes you might end up alone or raising someone else's kid. Because if the male-"partner" has the wealth and intelligence (which can be taught to some degree), but the pool boy/gardener/yoga instructor/one night stand has the genes, then the female's offspring theoretically can have wealth, genes and intelligence.

PPS./Disclaimer: I'm in the losing category, so this not incitement to eugenics, or anything like that. (besides, it does its job automatically) This is just an observation.