A (bi) girl cheated on her girlfriend with me. Further explanation down there, am I the bad guy?

Anonymous

We met at a club through mutual friends. We started talking, and she was really nice and had a good sense of humour. I was aware that she was in a relationship. A friend of mine and his more or less girlfriend went out to dance, at which point she also asked me to go to dance. She clearly made some moves on me, at which point. I admitted that I dont really know how to do this stuff. She laughed, and we continued dancing. She pulled me closer and closer, then asked me if I wanted to go some place more private. We went out to the garden and she started kissing me. I did not resist, which now I seriously regret. As I mentioned previously, I dont have much experience, so one of my justifications at the time was that I wanted to try out what I can. Again, I regret this looking back. She kind of showed how I can be a better kisser. Our friend interrupted us here, who know that she is in a relationship. After this, we danced a couple of times and then me and my friend walked the girls home. As me and my friends are walking home ourselves, I get a deeper and deeper sense of shame from what I did. I decided that at the very least I would apologise and tell her that it wasn't fair on my part not to stop it. I wrote her a message, but she hasn't opened it. Later I also noticed that she unfollowed me on instagram, and removed my following (her profile is private). I hate what I did, and if I can't even apologise I dont know what to do. So please, be honest, am I the bad guy in this situation? Because I certainly feel like I am.

Updates
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I left out that after we went outside and kissed, she told me that the girl I knew she was in a relationship with was far away at the moment. I dont really know if this is important info, just putting it out there.
A (bi) girl cheated on her girlfriend with me. Further explanation down there, am I the bad guy?
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