Threesome ended in a twosome

I was invited by a guy I hooked up with a few times over the course of a few years, to be his and his new girlfriend's guest of honor. He was the initiator of the threesome and she was aware of our past and claimed to be fine with us all having sex with each other. I was all for joining because I had the fun, easy non- emotionally connected role. But I had difficulty processing her allowing it because I would never allow my man to bang another chick- it's likely opening Pandora's Box for a couple. Plus, odd numbers in people frequently lead to one person feeling like a "third wheel." Both claimed no drama no jealousy and no one reported any boundaries. It all started out sexy, like a movie, but ended in his girlfriend being left out and forgotten while he was giving me anal (which he was always into during past hookups). Ultimately, I ended up forgetting his girlfriend was underneath me, and at some point she crawled out and onto the floor and watched him give me anal for an hour. We must have also forgotten she was in the room, I know I did. She became jealous, and apparently was turned off by anal, stating she couldn't have sex with him anymore tonight, essentially game over. It was a truly bizarre experience.