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My favorite rebuttal to that is " Oh.. A boyfriend? How long have you had that problem?

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I generally leave the girls alone with actual boyfriend, however that line has got me some decent action.
Haha nice. I'll probably use that line the next time a girl throws that at me.
For the OP, usually if she IS giving you that line that means you've come off too needy and shown too much interest, so she she playing that card as a shield to ward you off, regardless of whether it is true or not.
You can go sexually playful and use Fuze's line or you can also use what I do:
Laugh or chuckle as if she said something very irrelevant to the conversation thread, just make sure it's not in a mocking way, then say something like, "Woah did it come off like I was hitting on you? Because that was not my intention at all, you just looked like you were enjoying yourself and having a good time so I wanted to see what the fun was all about. I mean don't get me wrong, you're beautiful (she actually HAS TO BE beautiful but this goes without saying), but [I have a girlfriend/you're not really my type/I look at you as a good friend or sister even just by the vibe you give off] (all negs)"
Hope that helps. PM me if you have any other questions. Also be sure to check out the website below. The guy in the videos is my mentor, Arash Dibazar, and he's a Social Genius
All the best,
Sep