Just posting this in case people are curious...
Thanks for the posts everyone. She came over tonight and we sat down and I laid it out and talked to her about it. She came over upset because she knew she was wrong to go. She talked to her mom about it and her best friend and they basically called her an idiot (but using nicer words of course) for going. Her mom told her "You won't be able to pull something like that when you're married" which she took quite seriously and really made her realize what exactly she had done.
I know she didn't MEAN to hurt me as much as she did, or she acted like she did for no great reason (girls do confusing things), I know she didn't do anything and felt it was innocent. Some of you may think she's cheating 100% or I'm wrong or whatever, but I've known this girl for 3.5 years (sorry, dated for 3.5 years. I've known her since I was like 12 and she's always had a crush on me since we were little) and I know she wouldn't cheat on me. It's my call to decide whether I trust her or not and believe her. It is just a situation that isn't necessary to put yourself in because of how it can make me feel and the situations that can arise from it. She felt she had good judgement of the situation because she knew the guy but based on the information I had the whole thing seemed off.
It's a relationship and there's trust and respect and I believe they're essentially interchangeable. Ex. If I was confronted with 'you either trust me or not' question, I would give the 'you either respect me or not' question. I don't think it's as black and white as you trust her or you don't, there's give and take, etc.
Anyways she apologized, cried, I told her I'm telling her how I feel about this so it doesn't happen again and she got not only my opinion but also two people close to her who shared the same thoughts as me so I didn't seem controlling or unreasonable to her. She's only ever went to do this once so it was like a one time screw up. We make mistakes, I'm willing to forgive. Of course if it happens again then that's another story.
It's a hard thing to be concerned and show that and not come across as controlling, jealous or untrusting. I told her I trust her and I said it this way: If I didn't trust you, as soon as you left for the hotel room our relationship would have been over because really, who KNOWS what could have really went down. There's no reason you need to go there. Since I trust you and you just had bad judgement this one time, we're clearly still together. I think it's a valid point.
At any rate, it's better this came up sooner than later (if we were married, say).
I told her that I trust her but I also CARE about her. She put herself in a risky situation and I want the best for not just myself but also her.
Flame me or not for staying with her, but I'm giving her the benefit of the doubt and this wasn't worth ending the relationship in this case. A lot of people gave some good, rational thought out logical advice and I thank you for it!