Well that's intresting, I haven't heard one of those before
I think it is cheating, but it's the other person, and they usually don't see things as you see them. I say sit with her and talk (ugh yeah old fasioned talking, can't we invent a laser that will do the job?). And tell her how this makes you feel. If she is too stuck up to talk with you about this then I don't know. I'm sorry. But if she will listen to you, then tell her that it's still cheating. She is enjoying her experiences with another person at the same equivilent as to you. That makes you feel...how?
I would feel kinda cheated, because here is this person I devote time to and have fun with but she goes and has a second person
. In love, it's about selfishness for the person you are caring for.
But here's the thing: you can have a hot lesbian threesome
But the first thing I think you should do is talk with you girlfriend and tell her how you feel. It might feel hard to do that. But it isn't really that hard if you put it in your own words. You could start it out as "hey you know this girlfriend you have. Yeah well see..."
or maybe "I don't like it that you have another person your seeing. That makes me feel bad"
Those might sound kinda weird to you, so please put it in your own words.
Hope I helped