Listen up, do this.
This works because I've done it plenty of times, with success.
If she asks it's usually because you're being asexual beta male and not making moves when she's making herself available to you. I get it on occasion because I don't have a girlfriend and girls wonder why. (but the following advice is based on past experience, back when I was seeing almost a different chick every week).
Anyway, when she asks you, look her straight in the eyes. Make sure you are standing close to her, face-to-face. Slightly lift her chin and kiss her on the lips. No tongue or very little, just a sensual smooch, and then work it for a few seconds. Nine times out of ten she will like it. Then you look at her straight in the eyes and say "do I kiss like a gay guy?" She's going to laugh, and that's when you start making out.
I've done this at least seven times in the past. Try it. I honestly bet you she wants you, and the "are you gay" question is because you are not making moves like you should be. So you surprise her with a kiss like this.
And hey, if you don't get a chance to kiss her right then, you can pull her aside later. Be like, "you know the other time when you asked me if I was gay...well..." and then you kiss her. It's spontaneous, chicks dig it.