It's one thing if you were dancing and the guy just slammed you on the lips.  If you kissed back or provoked it, then yeah it's your fault.  Otherwise, I say this... If you were just standing in the store looking at clothes and someone walked by and smacked your ass or squeezed it while he was there, or even not there, isn't that the same thing as saying "if a guy every kisses you I'm done."?  You only have so much control over what other people do.  Being in a relationship doesn't mean you need to be a hermit crab without him.  Your other option was sit in the corner and not dance or have fun.  You were dancing and a guy decided to kiss ya.  You being torn up the rest of the trip says enough to me.  It wasn't you, it was the guy you danced with.  If he doesn't understand that, you two are gonna have a tough time with the hard parts of living together after marriage.

I woukdn't give it any more thought. You should have done it. You did it. You've learned your lesson and won't do it again. If you spend time feeling guilty, you actually end up taking more from your boyfriend than what has alralready happened. Don't tell him. If you feel that you shouldn't have done it then make it up to your boyfriend by treating him even more specially.