This is a tricky situation for sure. I know that when you meet someone who you’re attracted to and the feeling is mutual, the pull is so strong that it can feel impossible to resist. However, you are treading on thin ice. You need to get clear about what you want for yourself. Are you interested in a non-monogamous situation? Is this girl being honest with her girlfriend about you? If not, does it feel ok for you to be part of a deception? And how might this affect you and the relationship with this girl down the road? Take some time to think about what you really want, and then try to make choices that support this. If you want a monogamous, committed relationship then this may be a very painful path for you to continue on. But maybe you are ok dating a girl who is partnered. My most important advice is to be honest! Mostly with your self. As well as with the girl. There is a lot of potential for you to fool yourself into thinking this is ok with you, even if it’s not. Knowing yourself and setting clear boundaries always pays off down the road, even if it means the pain of letting this girl go. You deserve to have your needs met, and only you can ask for this.
Best of luck to you!