Does she feel bad when she kisses him, pushes herself against him like a wanton girl from a romance novel she's always pretended that she hasn't read?
Well. Yes and no, honestly. Because there's a part of her that's always been rather more attracted to Puck than Finn, a part of her that wants to sleep with Puck rather more than she wants to sleep with Finn.
Of course, she can't really get technical about it: Rachel Berry is cheating on Finn Hudson. Puck is screwing over his best friend. Again.
It's probably primal lust. Something like that.
And she's always been attracted to boys with piercings.