IMO, him actually telling you that it happened is better than if he didn't tell you. By him telling you, he's feeling remorse for the action. He also left the situation, so you know that he knows what he did was wrong.

I have first hand experience with this and when my ex told me that he kissed one of my best friends, I (not being properly treated for a mental illness) overreacted and felt the need to punish him by sleeping with someone he absolutely despised and my best friends ex. I would also like to add that by the point in the relationship that this happened, we weren't getting along very well, mainly due to the mania I was experiencing.

Anyways, whatever you do, don't overreact. Talk to him. Don't shut down. Remember that you have a solid relationship with him and that he cares for you as much as you do for him.

Also, I know when I'm at a bar and there's dancin', and loud music, and alcohol I tend to loose myself in the music and I start dancing with whoever is around. That would be why I don't go out to the bar......