I slept with my engaged friend…

This requires some more context because this is such an insane situation I’ve gotten myself into. I feel like someone is using a monkey paw on me where something positive will happen to me with really bizarre caveats.

Anyway, I’m 25M and I have a friend who is 25F. I’ll call her Anna. We’ve been friends since middle school but only really got close in HS. We also share a birthday which is kinda fun.

Anyway, I had a huge crush on her Sophomore year of HS. I asked her out 3 separate times and she rejected me each time. The first one should’ve given me the hint but I was stupid and tried two more times. Somehow I managed to accept 3 rejections and not let it ruin our friendship so it worked out.

Fast forward a few years, we end up going to college 15 min away from each other and hang out fairly often. After we graduated we also settled in the same cities so we still see each other pretty often.

Anyway, about a year and a half ago, she started dating this guy, Brandon, and then the three of us started hanging out all the time. I didn’t really mind, he had a weird vibe but was friendly with me and I never got a hostile vibe or felt like I was third wheeling. After awhile, they told me they were engaged but it’s one of those engagements where they don’t even have an estimate for the wedding date.

The three of us, and sometimes others, would continue to hang out regularly. Then in January, Anna and I went out alone together for NYE and I noticed it seemed like was flirting with guys in the bars. I thought this was odd but assumed I was reading too deeply into things.

We hung out again a week later and she told me that Brandon had requested that they open their relationship, seemingly just so he could also date this other girl. Anna told me that she reluctantly agreed to it.

For the next few weeks, we hung out without her fiance exclusively. And I also noticed that she seemed to be flirting with me but assumed I was reading too deeply into it because she was engaged and because she had rejected me before.

One day, she was hanging out at my apartment with my roommate and I mentioned that I finally got the posters I had ordered. She offered to help me hang them. I said “nah its fine” but she basically insisted. We go into my room and she shuts the door behind us and bluntly asks me “wanna fuck?”.

I asked if she was serious and she said yes. I asked why and she said she felt like she might as well take advantage of the open relationship but would rather do it with someone she trusts.

So we do the thing and it’s a bit awkward but ultimately a good enough time that we keep it up. And it becomes a pretty solid FWB thing for awhile. The entire time I never got the sense that either of us developed feelings for each other.

Anyway, apparently this whole time she had told him that she was also seeing someone and he didn’t care. However, this weekend was the first time she told him that the other guy was me and he blew up. He accused her of starting an affair before they opened the relationship and said she’s emotionally cheating on him, etc.

Now Im roped into the middle of this and I feel like I should have seen this becoming messy but I didn’t expect it to go like this.