From a man s perspective, monogamy is hard to understand. If the evolutionary goal is to spread our genes as widely as we can to assure the survival of kids who look like us, the more women a man can impregnate, the better his chances of living on in his progeny. From a woman s point of view, on the other hand, the better she can tie a man to the children she has borne him, the better their chances of survival and the more likely her genes will endure. In the majority of human societies, including ours, women have won the argument. Most are monogamous.
Some primates are monogamous, some are polygamous.
Those species, including homo sapiens , in which males tend to stay with one partner for a lifetime have half the number of circulating white blood cells of species in which males spread their seed far and wide. Messing around exposes individuals to a whole range of venereal infections. White cells are major weapons in the infection-fighting arsenal of mammals. The more exposure to venereal disease, the more white blood cells a body needs. Beefed up immune systems are apparently so necessary to the survival of polygamous creatures, that individuals of these species are born with high white blood cell counts and maintain them throughout their adult lives, even if they remain virgins.