I've encountered the same sort of thing--women on OKC approached me (and poly was explicit in my profile) and acted interested until it was apparent that my wife did know about what was going on, generally, and that I was communicating with them, specifically. There was a good deal of interest that got stifled at that point.
I suspect it has a great deal to do with those folks being unable to wrap their heads around anything other than dysfunctional monogamy. They've managed to rationalize their intent to cheat with somebody else's husband and have come to terms with that. Coming to terms with a poly entanglement is a foreign concept and more than they're prepared for.
Perhaps it's due to feeling safe that the relationship wouldn't get very serious because it would be an illicit affair and the man isn't likely to want to shack up or anything like that. They can get the passion and excitement they crave without the demands of a serious, daily relationship. When that potential relationship turns out to have the possibility of becoming a serious, daily thing, it's beyond their comfort level.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.