Instead of being angry at yourself, use the experience as a lesson for your future adventures. I'm sure that your boyfriend would not break up with you over this, it's really not that big of a deal. You assumed that the guy wanted to dance with your group and once you realized what was going on, you got away from the situation. You didn't give him your number or kiss him. See? That means you have class and character. You knew what was the right thing to do and once you unfroze from the initial shock, you did it.

But the thing that you can learn from this is that clubs are really not a good place for people in committed relationships. To be honest, I'm not sure how I would feel about my girlfriend going dancing to a club with some guy friend. To say the least, I would not be very thrilled. You did the right thing when put under pressure, but maybe the lesson to be learned here is to be more respectful of the fact that you are in a committed relationship and that it comes with certain responsibilities. One of those is saying no to places and events which could jeopardize the level of trust and respect in your relationship.