In junior high, negotiating relationships and ex-relationships was easy. Any boy who didn't want to be with my gal pals was a jerk and I didn't talk to them anymore. And any girl who dated my friends' exes was some kinda something. Things are different now and I sometimes remain friends with friends' exes. Things are really different in that I now have become friends with friends' exes new girlfriends and wives.
I like to think we're all mature (read: ma-toor) enough to be in the same room together, but human emotions get involved and sometimes that's just not comfortable, not least of all for me. There's definitely a feeling sometimes of being in the middle. Take the other evening when I found myself with a close gal pal and her old flame's new wife. I hadn't been together with them before, so I was very conscious of the hugs and kisses I gave new wife in front of old girlfriend. Though we used to poke fun at new wife together, I've since gotten to know her and found her perfectly pleasant and nice to be around, but old girlfriend wasn't really around for the getting-to-know part.
I've also become quite close with the ex-boyfriend and his new wife of another friend. This ex-boyfriend had made something, let's call it a 'birdhouse' for lack of any other imagination on my part, for his ex-girlfriend, my good friend. This 'birdhouse' was given or taken back in the break up but has landed with me several times in the years since. In my recent move in with A, I declined to take the birdhouse with me and another friend took it to put in her 'yard.' I'm not losing sleep over this, but I am aware of the fact that when the ex-girlfriend-friend visits the home/yard where the birdhouse is now displayed, she might have a reaction.
I'm learning to let people have their own reactions, to be open and willing to talk about them and negotiate ways to keep people comfortable within my own home or event, but not to take responsibility for managing these people in other circumstances. I've also stopped being afraid to say an ex's name in front of the wrong people - these are people in my life, so they may get mentioned, although I try not to do it gratuitously. And finally, I've learned that there are some advantages to my having kept my dates outside of my own social circle for the most part.