You're in a LTR. Things are great: she does whatever you ask, she's happy, you're happy, and she's never ridden the carousel (you're her first for everything). You have everything you want, and you know if you find you want something else, you can ask her, and she'll do it for you.

Then you find out she hung out with a guy who you don't get along with. She says she didn't tell you because she knew it would upset you. You know that nothing happened between them, and you know she isn't even attracted to him, but the problem is still there: she knowingly went behind your back.

How do you handle it?

Again, she will do anything you tell her to. Completely staying in the relationship just seems like asking to be cheated on. Completely leaving seems rash since you already have an ideal dynamic, minus this incident.