I think I would have broken up with him the first time he made it clear I wasn’t welcome in company with the two of them. Taking her to a weekend trip to an amusement park with a shared hotel room where you’re expressly unwelcome?
Come on, now.
On the other hand, this is coming from someone who’s husband went to a weekend research conference on the other side of the country, then, after the fact, told me that he’d shared a hotel room with his female lab co-worker, and was genuinely, honestly surprised that I found that particular detail of the trip to be noteworthy.
My husband is a hard-core, computer-gaming, yeast-genetics PhD nerd. He hasn’t one ounce of playa in his entire being. He has a few female co-workers which he’s become pretty good friends with, but I think he’d break out into hives and a cold sweat if one of them tried to hug him or something. I think if he he wasn’t exactly the person he is, and Claire wasn’t who she is, I would have practically divorced him on the spot. As it was, I was taken aback, but I really had to laugh about it. He was totally puzzled about why they should have wasted money on separate hotel rooms or been uncomfortable bunking with strangers, both of which were options. After all, there were separate beds and he slept in his clothes…? I suggested, as a guideline, that any time he slept in a room with a vagina that didn’t belong to me or a relative, it’s probably noteworthy. He thought that was fair enough, and I got to go around telling all my chick friends that my husband secretly spent the weekend at a work conference, sharing a hotel room with another woman… just to see their reactions. It was pretty funny. 
So, I suppose I should say, that it sounds completely sketchy to me. It’s not that he wants to hang out with her that bothers me, but that he doesn’t want you along. I should make it clear that I’m perfectly okay with NajaHusband hanging out with his female friends alone, but if I’m free, he’s always perfectly happy to invite me along, too. Your story sounds to me as though your guy is treating you with great disrespect and disregard. He clearly still has feelings for her, and wants their interactions to be just like the interactions they were twenty years ago. The problem with you being there is that it would mean they’d have to act like it is now, today, where they are both committed to partners.
Aaaaaaaaaaand… On preview, I see the thread has exploded. I’ll just go ahead and sum up my post with “DTMFA”. 