[jayjader]

Marriage trades the advantage of sowing wild oats for the advantage of a pair of committed adults sticking around to raise the children to adulthood.

Survival of the fittest is useless if none make it to reproductive age (or the opportunity to reproduce), and humans are incapable of fending for themselves for at least the first couple of years of their lives. Simply reproducing with many is not a surefire way, and doesn't seem to be the optimal way, of guaranteeing that your genetic material endures.

Bret Weinstein has some interesting thoughts on the evolutionary merits of things like marriage and religion, I'd reccommend having a look at his work.