While I certainly understand the limitations number crunching can put on a project, my first thought is: why have it only be about CURRENT relationships??
What if there was a survey done where people could identify as "non-monogamous for all relationships", "non-monogamous for most relationships", "non-monogamous for some relationships", "non-monogamous for a few relationships" and "monogamous for all relationships". Then you could include ALL people, whether currently single, in one relationship, or multiple relationships, as long as they've been in AT LEAST one relationship for 3 months or longer at some point. You'd get their thoughts on their relationships as a whole rather than one specific relationships (cause let's face it, things change, and the results you get now could be VERY different from the results you'd get in a year from the exact same people!) and it wouldn't matter if those relationships were serial monogamy or polyamory or open or whatever.
Of course, since I'm not sure of the exact point of your research, I could be talking out of my ass, but it seems to me like if you're trying to look at the personalities of those who get involved in poly versus those who don't, looking at their overall relationship style and attitude would tell you more than just their thoughts on ONE relationship.