I have a girlfriend of 5 years who recently told me she has cheated on me while drunk at a party.?

We've by no means had the perfect relationship, but neither of us wanted to break up with the other prior to the incident. Her story is that a guy that she knew in Highschool showed up to an all girl party when she was already drunk. She says that she doesn't remembers starting to kiss him. She remembers only some of it, but she doesn't remember thinking that what she was doing was wrong while she was doing it (which she presumably would normally). She also remembers bits and pieces of having with him. She remembers flirting with him when he got there. She remembers touching his bare back while they were in a pool before anything had happened. What I'm wondering is whether there are any girls who have had similar experiences. Have you ever been in a relationship that you didn't want to leave but got drunk and had with a guy that you otherwise wouldn't? I'm having tons of trouble with this because whenever I drink, I never get so drunk that I'm not somewhat in control. Thanks.