Do you mean that you're more likely to get a monogamous relationship with someone monogamous, and more likely to get a poly one with someone who's poly? Or am I missing your point?

Yes, but also that if you're primarily looking at people in the pagan/geek/bi cluster, you'll see many more poly people there than elsewhere (relatively, at least).

As for unicorn hunting, it's usually about being annoyed by hypocrisy, so far as I've seen. Bisexual women get annoyed when they have straight male partners who want the rules of their relationship such that the man can date whoever he wants, but the woman can only date other women. This is often derogatorily referred to as a "one-penis policy," and seems to be the result of the man not feeling threatened by lesbian relationships because he doesn't think of women as "real competition." (This is not a straw man. I have actually seen and heard this view stated seriously by some men in the poly community.)