Caught My Partner Cheating But Went Further Than I Planned

I'm a 35-year-old nurse in an open relationship. It's been great, freedom to explore and grow without the pressure of exclusivity. Last week, though, I walked into my place and saw my partner, Rachel, with another guy. She had explained earlier she was meeting up with him casually, but seeing it happen...felt different. I was angry, hurt, mostly because we agreed on honesty and clear communication. But instead of walking out or confronting them both, something else happened inside me. I got this rage-building feeling that needed an outlet, like the pressure was too much for my mind to handle. So, without thinking properly, I went over and joined in. I remember being so shocked at what I did right after. But then it turned into a threesome with Rachel's friend too. We were all naked on the floor of her living room, touching and kissing, me pushing back every thought of monogamy or what this meant for my relationship. It was like a switch flipped in my head, one that let go of control. Afterward, I left them both behind with a mess to clean up, feeling hollow but oddly calm. The next day, it hit me hard. Why did I do that? Is this normal when caught cheating on your partner? Did joining in somehow validate what Rachel was doing? It’s been eating at me ever since. I feel like there's something I need to understand about myself here. So, am I the asshole for reacting so harshly by joining them without consent, or is it justifiable as a reaction to betrayal? What does this say about my sexuality and relationships?