['I was so into that guy. I thought he was really attractive.', "Is there any chance for something to happen? Seems like you developed a connection from afar, if he's still on your mind. Why do you think there's no chance?", "I really shouldn't because I have a serious boyfriend."]
Oh.. well that complicates things. Yeah, maybe that will make you seriously examine things. I think you're okay as long as you don't do anything while still in a relationship. But if you feel your current one is lacking something, and your eye is wandering, you owe it to yourself and your current boyfriend to reflect on why that is, and if there can be improvements, or if you should both move on.
One time I was hit on by a guy I liked at work. I was really interested in him, and I said no because I had a boyfriend.