Im having a long distance relationship right now, my girlfriend and I are both going to college and we live far away from eachother ( I live in Canada, she lives in the US). I very rarely see her. We've been together like this for 10months.
She has always had a crush on this one girl in her area, they just recently became friends. She tells me she always fantasizes about her. Now that they're getting closer, she wants to experiment with her.
Im pretty upset about this, its like cheating right? Also, her and I haven't even had sex yet, i'd kinda want our first sexual experience to be together. Now if I was there, i'd allow it if I could join them, but I cant.
So last night we spoke on the phone and I asked her not to do anything with this girl. We argued a long time, I threatened to break up with her if she does anything with her. The thing is...She still hasn't decided if she wants to experiment with this girl or not, even after I told her i'd leave if she does.
I really dont know what to do. I like her very much, we really got to know eachother over the past ten months, we even spoke about living together after college. Is it really worth breaking up with her over this? I dont think so, but what more can I do.

Should I let her experiment? If so, Im sure it wont just happen once and be done with, it'll happen many times for sure...
Should I break up with her? I really dont want to leave her for this...

I dont know what more to do, I argued with her about it. I tried to guilt her out of it. I told her not to do anything with her, I even told her its cheating and ill leave her. But she still hasn't decided if she wants to or not.

Now this fantasizing has been going on for many months. She's not a lesbian, she's bi. She dosn't want a relationship with this girl, she wants to experiment.

What do I do?