Actually, since our Western societies practice Middle Eastern religion, I'll point out there is nothing in the Christian bible against polygamy! :p

And Fenix, as Josh, some cultures practice polyandry.

"In rural regions of India, Nepal, China (in particular, Yunnan and Tibet), and surrounding nations, fraternal polyandry, in which two (or more) brothers marry the same woman, is culturally accepted. Polyandry is also the accepted norm in a few African societies."

For some women, one man is not enough. So instead of calling them adultresses, why not let them have more than one husband?

If a person is "bisexual" why not let him or her have a husband and a wife?

Or if a man wants to be have more than one wife, that is not cheating.

Some women will gladly be with a man and another woman. They call this threesome, if it's just three.

And as Josh said, lesbianism is quite common in harems or when a man has more than one wife! (I guess the wives can't depend on the husband to get around to them all that time).