Wow, I love Lesbian porn as much as the next straight bloke but if it was my hypothetical gf getting off with another girl whilst we were together I wouldn't be thinking "ooo, awesome".
If I were in a relationship, I wouldn't kiss, mess around with, or screw anyone else out of simple respect for my partner. Unless you agree on an open relationship at the start it's assumed that there will be no one else involved, so bonds of trust form, trust to form an emotional or physical connection with that person. Regardless of gender, involving other people with both parties consent in a breach of trust and I would deal with it the same way, again regardless of the gender..."Nice knowing you, now kindly F*** Off!"
Of course people view things differently, and that's great for them, but as far as I'm concerned, unless we're both happy for the relationship to be an open one, any messing around with a third party, be it drunken, a moment of weakness or whatever, then it's done. No second chances.