I just read a very convincing social explanation for the reason for "real" marriage.
All human societies are built around some form of marriage.
The pairing off between men and women for the sake of family formation and child rearing is something even the most primitive societies honor and protect.
But many primitive societies practice polygamy, a system where a man is allowed to take more than one wife.
Societies that practice hunting-and-gathering — the original human economy that lasted 5 million years — are almost uniformly monogamous.
Those societies that have advanced beyond the tribal stage to become “civilizations” have (with the one exception of Islam) also returned to monogamy.
Why monogamy? Because it does several that things that promote social well-being:
Monogamy creates a level playing field by ensuring that every member of society a reasonable chance to mate.
It reduces sexual competition among males and provides a format in which they can cooperate on larger tasks.
It redirects men’s energies from the task of collecting multiple wives to that of providing for their children.
It creates the protective environment of the two-parent family, which is without a doubt the best situation in which to raise children.
The underlying problem of monogamy is this: although successful in creating better societies, I distributes its benefits unequally.
There are winners and losers.
The winners are:
1) Low-status males, who would otherwise be excluded from mating. In nature, they form the “bachelor herd” and spend most of their lives fighting with each other in status competition for the chance to gain access to females. With monogamy they get an equal chance to mate.
2) High-status females. Instead of having to share their mates with other females, they can claim the exclusive support of a higher-status male. That is why married women are usually the strongest supporters of monogamy in any society. (Although they also benefit, low-status males are usually too concerned with their low status to appreciate monogamy’s gift to them.)
The losers, on the other hand, are:
1) High-status men. They have their sexual horizons constricted and have to be content with only one wife.
2) Low-status women. This is often overlooked. With polygamy, a young, poor or unattractive woman can become the second or third or fourth wife of a more successful man. This is why, although polygamy is often portrayed as “oppressive to women,” it registers a surprising amount of support among women in any polygamous society. With monogamy, however, only by loosening its bonds can a lower-status woman become the escort, consort, mistress, Morganic or “trophy” wife of a more successful man.
Historically, most of the breaking of the rules of monogamy — “adultery” as we call it — takes place between high-status men and lower-status women.
Source: William Tucker's new book is Marriage and Civilization: How Monogamy Made Us Human.