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Being a guitarist myself, I would assume you played something the girls weren't interested in. If you were noodling or playing some band they've never heard off, they'll turn off.
Whenever I pick up a guitar in a social situation, I play something everyone knows, which gets everyone singing along and makes the girls more attracted. Learn some "Wonderwall" type shit and they'll eat it up
I started doing this recently in addition to singing and have been learning to do this with songs like Radioactive that are 4 chord (Am, C, G, D) patterns. These songs are incredibly easy and you can get a great response from people around. But good lord how many songs can you name that are 3 or 4 simple chord progressions. Too many... Take advantage of the simple things.

Rock on.