I think it's easier to talk about sex than to talk about feelings.
My poly life has actually developed out of my experience in an open relationship. (In a very queer scene, so it wasn't part of the swinger scene.) I found that I'm not really wired for casual sex, so I moved towards the polyamory model. The community of people I know who are poly also overlaps heavily with the open/casual sex/kink crowd, though I know some who are more conservative in their approach to sex.
I think that if you took a random sample of monogamous people who are available for relationships, some would be available for casual sex and some wouldn't be. Some who would like casual sex would not spend too much time in a FWB relationship because it would prevent them from being with a monogamous partner. Poly types don't really have restriction (depending on how they practice poly), so FWB relationships are just more likely to be an option.