A recent article on Alternet, For Many, Marriage is Sexless, Boring, and Oppressive:Time to Rethink the Institution? by Amanda Marcotte, asks the question:
Formalized marriage and monogamy began for practical reasons, unrelated to any religious notions of "sanctity". Once ancient hunter-gatherers settled into agricultural societies and ideas of private property and inheritance came about, socially sanctioned monogamous marriage began as a way to control women's sexuality so men would know which children were actually theirs. Polygynous marriage existed for the very rich, but the women in such marriages were still monogamous, though men were not. It is because of this original reason that women are punished more severely for infidelity than are men, as men couldn't be sure of who their children were unless women's sexuality was tightly controlled.
Religious insistence on monogamy was soon added, as it gave the force of law to a practical idea in societies where religious leaders were the law. "God said it" leaves no room for debate.
People did not marry primarily for love until around the 18th century. It was strictly a practical arrangement, a vehicle for joining powerful families for the rich, along with inheritance reasons, and to have a socially sanctioned partner to have children with and work together for survival for the poor. Love, if it happened, was icing on the cake, not the reason to get married in the first place.
People lived shorter lives then, so "until death do us part", did not include decades of the "empty-nest syndrome". Most people were lucky to live long enough to see the youngest child to adulthood. Life itself was harder and more survival oriented, thus people did not worry overmuch about love or personal fulfillment then.
Still, infidelity occurred all throughout history for both sexes, despite sanctions against it, as it's very difficult to overcome basic human nature. It's always been a big scandal for women, but not so much for men until the 19th century or so. The feminist movement no doubt influenced the increasing disapproval of male infidelity, rather than freeing women to male norms.
Today, we marry for love, life isn't strictly about survival, DNA tests prove paternity, overpopulation discourages large families, we live longer lives, women can support themselves, and the abolishment of legal distictions between marital and nonmarital children have removed much of the valid reasons for legal marriage and monogamy. Thus, marriage as it's currently understood has become maladaptive for modern needs. It's no wonder we're seeing what we're seeing.In light of this, marriage needs to be redefined if it is to survive in a workable form(s) and adjusted to reflect the realities of modern life and human nature. One of the first steps would be to cease mandating monogamy.