Just curious, Nerdy Guy, but did you just go out with this coworker the one time or is this now an ongoing relationship? I understand what you're saying, though. I also think because it's a coworker that you are friendly with (who is obviously respectful and knows you're married), that there was a good set of ground rules. Basically it was "I'm bored...are you bored, too? Great...let's go to the movies!"
I have a lot of women interject to discussions like this saying, "Oh, my best friend I've known since grade school is a guy. See, see...I have a platonic relationship. We've never done anything." Something tells me that the guy is either not attracted to the girl, or she has always been in relationships, and he's in SVP (Love that, by the way...so true!). Or he's gay -- either in the closet or out.