I have met many poly people the past couple year and there are a couple of patterns I am starting to recognize:

1) When a monogamous couple transitions to poly, it is usually the wife who is drives it, sometimes even demands it as an ultimatum -- and usually it is the woman who takes on a new lover first. Men have told me this transition is very hard on them... they will either stick it out or leave depending on how much they love their wife, and their own ability to fine additional mates. Most people I know outside of poly think it's the husband that pushes this idea of poly because he just wants’ to get his rocks off. They believe poly is mostly about sex. On the contrary poly is very much about "sex and relationships", not just “sex”, which is more in line with how women tick. Maybe that explains why.
2) I see that women in general tend to be more monogamous than men (with men). They stick with their man... until he pisses her off enough to leave him, then she’ll dump him for another. I have found in my experience that most women find the idea of having more than one man as a lover is absolutely disgusting. However there are a significant number of poly women who prefer this arrangement. In my experience I have noticed a number of these women that I know also identify them as bipolar. My wife too is bipolar and very interested in this lifestyle. I am wondering if there is a relationship between polygamous needs and bipolar. Don’t take this the wrong way. I am not trying to make the point that this is a disease that needs to be treated, in fact these women I know ARE treated bipolar, but it doesn’t seem to change their preference for poly. The point I am making is if this is sort of like why gay people are gay and straight people are straight. If it is something you are born with… my wife seems to be making this argument. I am having difficulty opposing it.

Agree or disagree... just some thoughts based on my experiences.