SUBREDDIT: r/relationships
TITLE: My [22 M] girlfriend [24 F] of 7 years drunkenly cheated on me.
POST: We were drinking in the backyard with some friends, and then she decided to go out to a local bar with her best friend. The bar was empty, but they met some random people that said they were down to have a party at their house. Even though multiple people left for the house, only 4 showed up (my gf, her friend, and two guys).

The two ladies were wasted by this point, and her friend passed out. Leaving out the details, my gf ended up having sex with one of the guys. She swears to having said "no" to hooking up with him the entire night but that she eventually "gave in" to his advances. She emphasized that it wasn't rape, so I don't know what to think.

This isn't the first time she has gotten way too drunk and done stupid shit, but this is definitely the most serious offense. I don't want to break-up with her because we've been through so much together and I don't want to go have "revenge sex" to get even with her. So what should I do?
TL;DR: 
GF got drunk and had sex with some guy. Anyone been in a similar situation? Any advice? What should I do?