Like Gandalf said, without knowing any of y'all, it's impossible to say anything for sure. But here's my 2 cents.

Anything is possible... worst-case scenario is, they are having an affair & knockin' boots whenever they get a chance. I say this not to bum you out, but just to put it out there. It would totally suck if that were the case, & it would really really hurt, but again there are far more horrible things in life. If you find out your lady is indeed cheating on you, then that's the end of that, she wasn't right for you & blah blah blah.

Best-case scenario is that they are, as she says, just good friends, & there is no attraction on the part of either of them. I'm a chick & I've had a lot of purely platonic male friends. That seems a little more likely to me than the worst-case scenario.

Now, in my humble opinion, it sounds like dude *might* have a little crush on your gf, especially if he's single. Everybody wants to taste the forbidden fruit, y'know? If this is the case, then your girl may or may not be aware of it. I can't always tell if somebody is interested in me; lots of factors go into one's perception of others' feelings & attractions. It's entirely possible to be blind to someone else's affection. And, like you said, she may be denying it just to get you to shut the fuck up. Odds are, she has eyes only for you & doesn't find this situation even worth discussing.

So, my advice is, don't stress about it. So what if some guy has the hots for her? As long as he's not seriously stalking her, it's not worth worrying about. Of course you're going to feel possessive & jealous; the important thing is not to let these feelings take over. There are few things more annoying than the over-protective boy(or girl)friend, & for most people, insecurity is a major turn-off. Just imagine you were in your girlfriend's position... how would you want her to respond?

Hope this helps.