My ex appropriated a large group of my friends while we were together. OK, I know people are allowed to be friends with whomever they want, and they did know each other vaguely before, but when I first met and became friends with some of these people, it was a couple of years before they really got to know him. Then we split but remained reasonably good friends, and he clearly saw all my female friends as his potential new dating pool, which annoyed the heck out of me. Most did not bite, but about two years later he announced he and one of my good friends (a woman I used to do lots of stuff with, though lately we've both been too busy) had developed feelings for each other, and they are now dating and very happy. And yes, they can do whatever they want but it still stung and irritated the living daylights out of me, and now I'm uncomfortable with her too. And this woman is kind of central to my entire group of friends - everyone knows and likes her, and she is part of all subgroups of friends I enjoy spending time with as well.
I do not wish to date him again, nor do I spend any time at all mourning the relationship or thinking about him and her, but I do NOT want to show up at a social event and see them there together. It may be irrational, but I am just so annoyed with him (and her, a bit, though I think since it's been so long since she dated anyone and since I know him well enough to know he's only shown her his good side and charmed the socks off her, I can't say I can blame her). I am annoyed because now he gets to contaminate my entire social scene with their presence together, which frankly I find just gross. Gross because I don't want to know what my friend's boyfriend is like as a Scrabble player, and gross because I find swapping partners within the same group of friends just weird.
So now I'm left with a choice of being uncomfortable in social groups I've known for years, or staying home. Gee thanks.
I was friends with all these people before he was, though he is now claiming he has known her longer than me, because he met her once or twice in a group setting about 3 years before I came along.
So actually yeah, I think I would want to be given the heads up that THEY would be there, or even if just he would be there, if I was there with a new boyfriend. And if he asked if I minded and I said yes, I'd be ticked to find them there anyway.
I guess my point is, why does Sally need to go hang out in THAT particular crowd now?