This is a fairly large issue for myself, being a single poly girl who would love to have a kind of partnership that involves merging lives and families and possibly raising kids. I posted a thread about my experiences back when I first joined the boards.
I've found that most poly communities are dominated by couples who wish to "open up" their relationships, which sets many limits for any relationships I can have from the get-go. And also, poly communities tend to view single people in very limited ways. (not everyone, but a general experience of mine)
I have been involved with a partner who is married and am grateful that I was open to that partnership. I found it to be very fulfilling and wonderful. (It's stated in past tense because it had to be put on hold with my temporary move overseas) While I'm open to such loving partnerships, even with their limitations, it does not change the fact that I would like to have some relationship that involves a deeper intertwinedness. However, the beauty of non-monogamy is that I don't have to expect that need to be met by every partner I have.
But also, I find with those specific needs (home, family, marriage, kids, etc) it is a hell of a lot harder to find people who might be able to meet them who are already poly (since most of them already have their own partner that meets those needs).