Hi ~ I agree with hamster. You haven't done anything that could be considered cheating. All you've done is seen a movie. There's nothing wrong with that.
I also agree that you need to be upfront with your boyfriend because certainly someone is going to tell him that they saw you with this other guy. And if you start hiding things from him, he's going to think that you HAVE something to hide, and that's not the case. So be upfront. Tell him the truth. Then he's going to trust you, instead of thinking you're slinking around behind his back.
The way you talk tho, I think you're a bit young to tie yourself down to one guy. I think you should just "play the field" and experience more fun before settling down to just one person in a long-term relationship right now. You have plenty of time to do that. From the way it sounds you haven't really dated alot - it that true? Now is the time to do it, because in later years it isn't that easy. So have your fun NOW. Get it out of your system while you can. I wish you the very best. God bless and take care. Hugs, Lee