RE: "Thanks...but I already have a girlfriend...."

In response to SlowLearner's dilemma: 

First, I think you handled the situation pretty well. You didn't lead her on and you were polite in letting her down. If I were in this cute girl's sassy pumps, I might've just assumed you were a little dense and didn't actually get that I was trying to ask you to dance (I mean, The Pause is pretty significant - obvious, even-  but there wasn't a direct "Hey-I-think-you're-sexy-do-you-want-to-take-me-for-a-spin?"). If not dense though, I probably would've figured you just weren't interested, but at least it wasn't a harsh rejection.

If you encounter a similar situation next time you're all up in the club though, I might advise a slightly different way to handle it. The trouble I see with just saying "Well, it was nice to meet you" and turning away is that the cute girl has no idea why you decided to turn away. Strapping on her sassy pumps again, I imagine that if I were standing there, staring at your freshly-turned back instead of gettin' low to the trashy music, I'd be wondering a lot of things: Nice to meet me? Yeah, sure, I bet. Well if it was actually nice to meet me then why don't you want to keep talking? They (the "royal they" aka Cosmo) always tell me that guys are always up for it!!! If I'm throwing myself at you, why don't you want me??? So what, not your type? Too short? Not cute enough? (How am I supposed to know you think I'm a "cute girl"?). And if you have a girlfriend, why didn't you just say so? That's the easiest way to turn someone down...I don't understaaaaaand! [NOTE: girls often over-think everything they hear.]

Emphasis on the last bit. Seriously: as a girl, if a guy is going to reject me... well, personally, I'd much rather hear you're in a relationship because (assuming it's true), it means there's nothing wrong with me, per-say... you're just already taken. It lets you off the hook, and it saves me from having to wonder what went wrong.

Unfortunately... it gets a little tricky here. You asked if you should just straight up tell her "Thanks, but I already have a girlfriend"... and I'm going to go ahead and complicate this. It would be an okay response, and she'd probably appreciate knowing that you're in a relationship. I'd probably prefer it to you just walking away without any excuse. However! the problem with telling her straight up like that is you are then drawing attention to the fact that the cute girl was hitting on you, and that she was unsuccessful (!). Since girls don't typically go out on a limb like that and boldly approach guys to start hitting on them, you don't want to embarrass her by obviously letting her know that you know she was hitting on you. In other words... if she doesn't explicitly hit on you and it's all implied (no matter how obvious it is to you), if you just go ahead and say "Oooh sorry I have a girlfriend you should back off", then you're essentially saying "Even though you didn't actually say anything I know what you are really trying to do and it's not going to work". And (at least for me anyway), that'd be pretty embarrassing.

But don't fear! The situation is not without hope! An easy way to get around all that would be to simply say something that lets her know you have a girlfriend, without blatantly saying "I'm turning you down because I already have a girlfriend." For example: "Hey, it was nice meeting you, but I have to go. I promised my girlfriend I'd meet her in a few minutes." Tada! These may sound identical to you because they are offering the same information... but it's all in the delivery. Again, at least speaking for myself here, I'd very much appreciate that subtle difference since it'd save me from getting too embarrassed.

To sum: SlowLearner, I do think you handled it well (at least from the way you tell it, you didn't come off as a real jerk). Telling her straight-up next time that you already have a girlfriend would honestly be fine, too, but the subtle moves might be the most considerate way to go about letting her down.