Pithy and perceptive replies. As a Christian, I believe it is morally wrong myself. However, there is quite a lot of polygamy in the Old Testament. Mostly for kings, of course. Men can be such rationalizing pigs.
Maybe an "open" marriage can work for certain kinds of people -- good point about it's usually one partner who is all for it and the other is dragged along. It's the transcendental knowledge of each other that I don't want to share with another person. Plus the STD's of course.
Thanks for discussing it a bit.