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Hmm, a few things leap to mind...

One, this guy does NOT seem like he's interested in a "relationship" with her, he wasn't before, he didn't keep in touch with her over 2 years, and it doesn't seem she tried to keep in touch with him either.

When she said she likes the fact that you're open about sex, is she talking in a relationship, or the "open relationship" thing? It's a different position when you consider yourself "taken" what you'll do as it is when you consider yourself "available." If you haven't, ask her how she'd have reacted if she'd been in a relationship at the time, and how she feels about his idea of open relationships, that should tell you a lot about how she feels when she's committed to someone.

Two - the "guy of the moment" thing - is she maybe concerned that you won't last after her last experience with a younger guy? What were the circumstances of their breakup? If she doesn't have any confidence in herself at being able to hold on to someone permanently - she may well have adopted an "enjoy it while it lasts" attitude rather than being devastated every time things don't work out; that can be part of having a poor track record for long term relationships, you give up hoping for "forever" and try to appreciate what you have while you've got it. Of course, it doesn't exactly WORK that way, generally it masks some insecurities and hurt from not being able to "succeed" there, but I've heard it from guys (and girls) who are afraid to hope for too much.

That's all that comes to mind at the moment, I'll add more if I think of anything!