Yes, I found out they had sex shortly after they did so. We were casually dating at the time and neither of us thought we were going to get into a more serious relationship, so it didn't bother me at the time. We didn't get serious for another few months, and he told me at the point that sleeping together had truly closed that door for him.

I have made it clear as we've gotten more serious that I'd prefer day-time meet ups and would like to be consulted before they land on plans. She's always the one to ask to hang out and hasn't invited me (nor has he asked to bring me) so it's been just the two of them. Honestly I'm really open to friendships with exes so I tried to be flexible, but after hearing that he still considers her a possible romantic option I'm not comfortable with them keeping a connection even if they brought me with them every time.

Honestly, he's very self-aware and seemed very careful to not get more seriously involved with me until he felt certain he was over her. That's why I was okay with their spending time together initially. But now hearing that he still sees her as an option, and knowing that he considers saying yes to her invitations important because he still cares whether she thinks he's an asshole or not, I'm just not feeling like I can be okay with it. I wish I could but I can't.