[ahimel]

Since I think it might be relevant to this thread, I'm female.

I can't imagine a serious polyamorous relationship working long-term unless each pair of partners really likes each other. So if you have two girlfriends, sometimes you'll all 3 be together, but sometimes Amy will have to accept that you want some time alone with Beth (not necessarily for sex, but sometimes it's nice to have a 2-person date, or just talk), Beth will have to accept that sometimes you want to be alone with Amy, AND you will have to accept that sometimes Amy and Beth want "girl time" to discuss clothes/watch a chick flick/dissect their dates/discuss menstruation, etc. It doesn't matter whether or not their relationship is sexual independant of you, but it does matter that they have a relationship totally unrelated to you.

Otherwise, you're essentially trying to have 2 relationships (or more!) in the space where most people have 1. And most people with 1 don't have enough time to do it. If you have to build up 2 completely seperate relationships with completely seperate date nights, sex nights, dinners, movies, dances.... they'll never be satisfied, and you'll never sleep. But with 3 of you within a single relationship, you have enormous flexibility. If Amy has a painting class, you and Beth don't have to sit around bored waiting for her to get home. If you want to play basketball, Amy and Beth can join or not without feeling the slightest guilt about how it will affect the relationship. You have 5 possible combinations of alone-together, instead of the normal 2.