A Night of Unexpected Consequences

I started dating my girlfriend Anna when I was in high school, and now we're both starting law school. We've been together for five years, and recently we decided to try an open relationship. This wasn't something either of us wanted lightly, it took a lot of soul-searching and long talks before we agreed on the terms. For one, we established clear boundaries: no emotional connections with others, only physical encounters, and we would always be each other's priority. Last night, I went to meet someone new at a bar down the street from campus. We exchanged messages for days, and finally, I felt it was time to see her in person. Her name is Mia, and she's a few years older than me, very confident and adventurous. She's also incredibly beautiful, long dark hair, green eyes, and an athletic build that's accentuated by the tight jeans she wore tonight. We met up at ten PM and talked for about half an hour in the bar. I could tell she was interested from the moment we locked eyes. The tension between us was palpable, but respectful, no hasty moves or unwanted advances. We agreed to a hook-up session at her apartment not far away. When we got there, Mia led me straight into her bedroom without any unnecessary chitchat. Her place is small but cozy, with touches of sophistication here and there that give it a charming edge. She kissed me deeply, pushing me back onto the bed where she straddled my waist. The next part isn't easy to write about, it's one thing to experience something in the heat of the moment; another entirely to recount it later on paper. But here goes: Mia and I didn't use protection that night. It wasn't a lapse in judgment per se, but rather an impulsive decision borne out of our mutual desire to be closer together. She whispered something about being spontaneous and enjoying the moment without boundaries. I returned home around midnight. As I lay in bed next to Anna, my mind was swirling with conflicting thoughts: exhilaration mixed with guilt. Our open relationship was meant to expand our horizons, not lead us into potentially reckless behavior like this. Today, as I write this confession, I'm left wondering if we made a mistake, did I betray the trust we've built over years? Or did Mia and I simply indulge in something that happened to be against our agreement without meaning to? This confession is my way of working through my guilt and confusion after last night. What do you think about what I did? Am I an asshole for not following our rules?