I know this is a very obvious red flag and i know i should run as soon as she asked to be with me but i guess i need a RP answer.

Ive known this HB8 that goes to my same gym since the summer but i was too pussy to do anything about it. Anyway, she came home for the holidays and I told myself i wouldn't mess this up again so i immediately started talking to her and said we should get a workout in together and got her Snapchat.

She then messages me about 2 hours later to ask when we're gonna get that workout in. We start flirting back and forth and leaves the part where she had a bf until the end. That still didn't stop me because we were just gonna workout. After we finish, she asks "what are we gonna do after the gym?" So i take her out and within 2 hours she's stroking my dick. We did practically everything except fuck because "she didn't wanna cross a line and what we were doing wasn't technically sex" (hamstering).

Long story short, now she's telling me that when she's with her bf all she can think about is me, even when she's having sex with him. So she is contemplating breaking up with him to be with me. I told her that i wasn't ready for a relationship and she said it's ok that we will "see where it goes." Part of her wants me to tell her to break up with him but I know that by (almost) fucking me while in a relationship, she won't hesitate to do it again with someone else.

I guess my question is, how should i approach this without making it a bigger deal than it needs to be? I'd like to keep her as a plate but she doesn't seem to want that.