What does it mean when guy smiles at you when he has a girlfriend?

What you have to remember, is that guys, no matter how good boyfriends they may be, will always be at least a little bit attracted to other girls. This guy may just think you are attractive, but may still like his girlfriend better. However, there is the chance that he really does like you more, but if he's still with his girlfriend, it's really his problem, and interfering will not end well, no matter what. So basically, as long as he's with his girlfriend, just act like friends, and do nothing more.