Sorry, I should have also mentioned that she has suggested that, for the sake of their relationship, the two of them may need to be exclusive for a while. He's said to me that he wouldn't agree to that, and I've made it clear that I'd be hurt if he did.
Over the last couple of months, she's been pushing much more "current-relationship" discussion with him than I have. I've been trying to let him take the lead as far as when to move into "thinking about us" and out of "getting over her." ("Her" being the ex, not either one of us.) She's more aggressive than I am, and I think that's actually benefitted me, since he and I are at a level now that I don't think we'd have reached without her pushing the envelope in their relationship. I'm thankful to her for that, and for the ways she's taken care of him that I can't.
SchrodingersCat, your point about trying to force the relationship into a particular mold is a good one. Also about letting things be what they are and seeing how it works. I feel that I could only be comfortable, in the long term, in a relationship where I was fairly close with all the SOs and OSOs and what-have-yous, but who's to say, since I've never done it? I have a tendency to first try to figure out how I'll feel about something, and then later try to conform my feelings to that guess, in some misguided attempt at consistency or predictability. So thanks for that reality check.
I guess the only ongoing question I have, then, is how do I deal with the other woman's insecurity/jealousy toward me? I'm frustrated, because I feel like if she just met me, she'd see that I'm not some doe-eyed na´ve who's going to flip out and create drama when reality hits... or some scheming manipulator out to grab the man and keep him for myself. But she doesn't want to meet me, so I don't know what I'm supposed to do.