If you have no other reason to be concerned and have a stable relationship, just try to look at it as a sign that he trusts you. He was honest and open about an attraction. We all have them. Just being attracted to someone doesn't mean we'll act on it. I know that's easier said than done, but just try to think of a time when you saw someone who you were attracted to physically.....didn't mean you were going to act on it, right?
I think that open communication cures a lot of ills, and it seems like a great sign that he felt comfy enough to share with you.