A difficult problem?

So, there is a problem I am in at work. I'm a bartender at this place, and there is this guy I instantly had a cruch on. I am 100% open, but I couldn't tell about this guy, so I kept my mouth shut. Instantly he took a liking to me (helping me out, having a beer after work, general after work stuff) and doesn't really do that wih the other workers. I had him pegged as a definate potential, but I just found out he has a girlfriend, which threw me for a loop. I met her, but he asked me tonight if I wanted to go somewhere tomorrow, maybe "grab a bite, and hit the club" How should I take that, and how far should I take this offer?