Yay, more social scientists! Spread the love!
I can totally relate to what you are writing about. I come from a culture (no, not speaking of the white Finnish mainstream culture in general atm) which emphasizes familial closeness and where co-parenting isn't some hippy alternative way of life but rather commonplace. So my interest in poly is very family-driven most times.
I guess many people think of one man-two women as something that is primarily sex/threesome-driven, when what we do most of the time is hang out with the kids. For me, a big motivation is to help out my gf and her hubs with child/home care. Chaos-management is just that much easier with three adults. Also, opportunities for sex and romance are not that hard to come by when there is somebody to babysit every once and a while.
Eventually I think everyone's planning to live together and combine finances - so one person could take periods out of work easily to pursue other interests, and still there would be a double-income. I think that three adult-families actually serve to break some of the male breadwinner-female homemaker rut many couples are so frustrated with.
Me: bi female in my twenties