My girlfriend slept with her best friend which happens to be a woman. What are your thoughts?

I have a question on how you or what you would do in my situation. I have been seeing this woman (45) for 6 months. I am absolutely in love with this woman. We have talked about getting married and moving in together. I am male 38. Her bestfriend from her childhood, since age 10, recently moved back to town. The situation is this: I haven't ever met her best friend so I said bring her over to my place so I can meet her. I had one of my best friends over as well. We all got pretty drunk last night. The girls went to the bathroom together, as all woman do, but were gone for an extended period of time. I was wondering why so I went to my bathroom and my girlfriends clothes were off and her best friend (girl) is eating her out. My girlfriend and her were extremely drunk. My girlfriend comes out and is saying sorry in her drunken manner. She must have apologized to me 1000 times all night. She said, you are mad at me and I said, no, just disappointed and hurt. When I tried to talk to her about it, her response was, it is her best friend since she was little and it isn't a big deal. Then my girlfriend would try to kiss me and I turned away, so her girlfriend started making out with her in front of me. Keep in mind, my girlfriend and I have talked about previously whether or not she has slept with another woman before and her answer was no. What would you all think about this? I'm not looking to have a threesome nor are both of them. I look at this as cheating. Need advice on what I should say and how to talk to her. Wht would you all do? Feel free to just comment with multiple polls, because it will only allow you to choose one.
considered cheating and I should break it off
Vote A
talk to her more about it when she is sober and then decide if I want to stay in the relationship
Vote B
let it go, as to it could be a one time thing because she was curious
Vote C
Her best friend is jealous of our relationship and wants us broken up.
Vote D
Select age and gender to cast your vote:
+1 y
Do you think she was just experimenting?
My girlfriend slept with her best friend which happens to be a woman. What are your thoughts?
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