I(23m) found out my gf(21f) slept with her longterm friend(22m) and lied of omission about it. Am I wrong to feel betrayed?

Ok, short summary. My girlfriend Davony and I have been dating seven months. She's great, super sweet and smart. So great I was thinking of asking her to move in with me when my roommate moves out in February. This is my first "real" relationship and I'm at me knees in love with this girl.

Her only downside is Chris, her best friend. He is 6' tall, part hispanic with blue eyes that every stranger who meets him comments on and once modeled underwear for a local store.He is also good at every damned game or athletic feat in the world. Fuckin' Chris right? Chris and her spent a few years in the same foster home and are very close. Sometimes I think he crosses boundaries but she usually takes my concerns very seriously and talks to him and he listens. So, I haven't had too much concerns there. My one assurance has always been her telling me that they "never dated".

But tonight we were drinking and started playing I've never. We get to the end and I'm down to a finger. My friend smirks and says "I've never had sex with Davony" to knock me out. Low and behold, Chris-of-the- "he's like a brother"-to-me puts a finger down.

My friend calls him out. "Woah, I thought you guys grew up together."

Chris just laughs "I had a fucked up childhood, it was some real flowers in the attic shit, huh, Davey(his nickname for her)" and tosses an arm over her. She excuses herself to get a drink and avoids eye contact.

The night goes downhill from there. The air is cold and everyone finds a way out, except fuckin' Chris and my roommate. I confront Davony about it in the kitchen and she says that it was a long time ago and she couldn't explain it so she didn't try and that she was only fourteen and her best friends don't even know- that she keeps the past in the past and they had a horrible foster home and it wasn't romantic (they didn't date...so it didn't count? Shake my head.) but for comfort.

I assert that as her boyfriend I expect honesty, especially when it comes to sex and past encounters because sex makes me feel vulnerable (stds, fidelity, etc). And she kind of shuts off, like she does in any fight. So I try and keep talking. Chris bustles in and gets in my face and we end up fighting, somewhat. My roommate breaks it up and Davony says that she will ride home with Chris.

I don't know if I overreacted. Probably. But I still think this is a huge issue? Especially because she left with him? Advice?

tl;dr I(23m) found out my gf(21f) slept with her longterm friend(22m) and lied of omission about it.I found out during a drinking game. I confronted her and she made up some circular excuses and shutdown. Said dude intervened. We fought and she left with him. Am I wrong to feel betrayed?