Mijn partner vriend is altijd seksueel betrokken bij mijn vriendin als ik weg ben

I've been having a tough time with my girlfriend. Every time I leave her, her friend seems to get involved. This guy is always sexually involved with my girl when I'm not around. It's driving me crazy. I've tried talking to her about it, but she just brushes it off. I'm starting to feel like I'm losing control over my relationship. Today, I decided to confront the situation head-on. I went over to my girlfriend's place and caught them in the act. The guy was pounding away at my girl, completely oblivious to my presence. I was shocked and hurt, but I couldn't just stand there and watch. I intervened and put an end to their little show. Now, I'm left wondering what to do next. My girl is acting like nothing happened, but I can't forget what I saw. I'm feeling betrayed and confused. I don't know how to move forward from here.