This is a really interesting and complex topic for me. I've been meaning to post on this thread for a while but couldn't pull together the time. (Now I'm home with a cold and nothing else to do!)
I might be making a bunch of short posts on this thread today...in between getting up for more Kleenex and cough drops.
Anyway, the #1 reason I am non-monogamous is because I like having relationships with men who would not be compatible with me in a primary-partner / lifetime-commitment way.
For the last 7 months I have been with an awesome guy with whom (by mutual agreement) I am not compatible to live with / share finances / marry.
I am introverted, need a lot of time alone, and don't want to live with anyone or have a life partner or have kids. He's extroverted, needs a lot more sex than me, is kinkier than me, wants to have kids someday, and is looking for a primary partner to share a house / finances with.
He and I have totally different ideas about money management & careers, which would be a dealbreaker if I wanted lifetime monogamy or even a non-monogamous primary partnership with him. We would fight constantly about money & jobs. Ugh! We also have wildly different views on politics, which would irritate me eventually.
So that's how we're incompatible. But we're also compatible in many significant ways. We have similar views on non-monogamy / poly. We have almost identical taste in books & geekiness. We connect really well intellectually. His extrovert personality is a good balance for my introvert nature, and vice versa. The more we get to know each other, the better friends we are becoming. We are on the same page as to what we want with each other. We enjoy telling each other about other dates we go on. Being with each other feels really comfortable and easy. Oh, and we have really good sex
But if we weren't both non-monogamous, we'd be wasting our time with a relationship that's incompatible with our respective long-term life goals.