Monogamy is a system where each person has only one sexual partner during his lifetime.
Why was monogamy invented?
Monogamy reduces hypergamy.
Men want to have sex with many women. Women want to have sex with the most attractive man and only the most attractive man. In a society that has no restrictions on sexual relations, the most attractive man ends up having lots of sex with many women. The women are satisfied this way, so they have no reason to want to have sex with the other men. This leaves the majority of men incel.
To solve this problem, monogamy was introduced to society to offer a compromise between male and female sexuality. In a monogamous society, each man gets at most one woman, his wife. Since the most attractive male may only have sex with his wife, the women will have to move on and marry other guys if they wish to have sex. This balances sexual power and makes it much easier for less attractive men to get a looksmatched girl.
The fate of monogamy
Monogamy has largely ended in the West, following the sexual revolution. Proponents of the sexual revolution preached that there is nothing wrong with premarital sex. Now that premarital sex is accepted by society, inceldom has skyrocketed.
Male sexual fidelity
The basic concept of marriage is that the man invests in the family in return for the sexual fidelity of his wife. This way, the man can invest in kids who he knows are his. It is well known that women prefer men who are successful with other women. The evolutionary benefits are obvious. A man who has the opportunity to spread his genes to many women will probably produce sons with the same opportunity, thereby also spreading the mother's genes. This would indicate that women would actually prefer to marry men who can have sex with other women. Only in modern times has adultery included extramarital sex of husbands. Again, this makes sense in evolutionary terms. The crime of adultery is to cheat a man into raising children who aren't his. The basic concept of marriage is a contract where the woman pledges her sexual fidelity to guarantee that the children will be her husband's, and in return, the husband pledges to support the family.
Society without enforced monogamy
8,000 years ago, only 1 man reproduced for every 17 women who reproduced.