(23F)(22M) what do I do

My gf (23 F) and I (22M) have been together for about 3 years. Last January, I broke up with her due to some really shitty personal stuff I had going on. We got back together in June of this year where she confessed that she had sex twice with one of her girlfriends, boyfriends, best friends. (I know that may seem confusing, sorry). I decided to put that aside because I do truly love her but now we have this issue. Every single get together that she has, that guy is there because he is so close to that friend group. Halloween, there was a party and the guy was supposed to be there. I went and he didn’t show. Thanksgiving, the guy was there but I told my girlfriend that I did not want to go so she went without me. My girlfriend recently moved into a brand new apartment and is having a NYE/housewarming party, and he will be there. I have expressed my feelings towards her and the situation and no matter what I do, I’m somehow in the wrong for feeling the way I do. If I don’t go, then I get ridiculed for not wanting to ever hang out with her friends, and, this guy will literally be at HER apartment which is like HER living space and I won’t be there (seems idk weird to me). If I do go, what? I’m supposed to walk up to him and say “Hey man, so you are the guy who fucked my girlfriend, nice to meet you”. What do I do? I KNOW that if the roles were reversed she would be fucking fuming and not allow me to go or be absolutely furious with me. I feel like out of respect they (my gf and her roommate) should just tell this guy not to come. I feel disrespected and degraded as a man, that I am supposed to go to a party and be in the same room as someone who had sex with my girlfriend. I understand we weren’t together, and thats fine, what happened is the past, but I don’t want to have to SEE this guy.