There's been an expectation of monogamy from women for a long time, where the same expectation hasn't traditionally been placed on men until recently. It seems to have developed among the wealthy class, who needed to ensure their heirs were truly of their line. Therefore, women were expected to be monogamous so men could be sure they were the biological father of their children, while men were not held to this same standard. Even in medieval Britain, the men of the noble class were expected to have mistresses as a matter of course.

Of course, women got some of their own action on the side, despite these expectations. They just had to be stealthy about it. A recent DNA study on the Y-DNA of Richard III (the DNA handed down only on the male line) shows two breaks in it between Richard's most distant male ancestor for whom we have DNA, and his most recent male-line relative....which means at least two women over about 700 years had children in that line who were not their husbands'!

I would like to see poly marriages be legalized next, after same sex marriage becomes the norm. My husband and I and one of his female friends did an experiment a few years ago to see how people really feel about poly relationships, as we know some same sex couples who are against poly unions. The three of us went walking through the mall, with me holding one of my husband's hands, and his female friend holding his other hand. You should have seen the dirty looks we got! People even grabbed their children and moved them away from us. It was insane.

Personally, I'd love to have a sister wife. Or, to not be married at all, but in a casual, no strings relationship. I think marriage and monogamy puts too much pressure on most relationships, and contributes to unhappiness within those relationships. Naturally, some people may do very well with monogamy. But, it's not for everyone.