Hey all so I posted not long ago about some disrespect from my LTR girlfriend and some suspicions I had about one of her guy friends. Everyone was very brutal and honest and I really appreciate that. Her and I ended up talking things out and I legitimately believed we could work through things. You can check my post history for details.

So now, something has come up, and I'm about ready to pull the plug. The TL;DR is this question: is it as bad, still cheating just the same, if your girl does it with another girl?

I've always suspected my girlfriend had lesbian tendencies because she often comments on how attractive and beautiful different woman are, sometimes makes disparaging remarks about men in general ("all men are just..." kind of thing) and has initiated a threesome in a past relationship with her, her boyfriend, and another girl. This weekend her lady friend came over to the house. They were talking and drinking, I was giving them their space to talk and was just in another room doing my own thing. The friend was planning on crashing on the couch. I went to bed before them, but couldn't sleep. They're on the couch getting cuddly, and after a while I hear kissing, like making out, then moaning. It must have lasted maybe 10 minutes, maybe a bit more. I didn't go out to join them, but I wasn't invited either, they didn't come join me in bed or anything. I just stayed in bed thinking "WTF?" They thought I was asleep.

Girlfriend comes to bed, she's drunk, acts normal, cuddles up to me. The next morning she asks if something's bothering me and I tell her that I heard them hooking up. The whole day she's apologetic and crying and saying how she doesn't want us to break up, asking for another chance. It's weird... it being with another girl, it didn't feel bad, really... but on principal I believe it shouldn't be viewed any differently than if she had hooked up with a guy. The issue for me is that she didn't explain beforehand that she wanted to experiment with another girl, we didn't agree on any kind of open arrangement like this. She went behind my back, thinking I was asleep, in my own place.

Would this be a deal-breaker for most of you all? She feels awful about it, is claiming that this was the first time she's every been unfaithful like this, and that it will never happen again (okay... ). I'm just thinking that, if she could do this with a girl, what's to stop her from doing it with a guy in the future if I forgive her? The issue is trust and having done this thinking I wouldn't know. To me, it's cheating, and I shouldn't stand for it. Anyone on here go through something like this? Is it somehow less of an issue for you if she does this with another girl?