Tell em "Yes, I'm gay, but I want to change and I'm looking for a girl to help me." Then tell them you've never had sex with a woman before.

I usually answer like this:
HB: Are you gay?
Me: I can prove that I'm not ( with a nice cheeky sorta smile )

The question can be one of two things, either the HB is interested in you, or she interested in why you look or act gay. Which isnt a bad thing my confidence is often construed as gay but heres how i handle it. Dont EVER say yes cause then theyll be pissed when they change in front of you or youre gonna hear how she blew some random bro in a parking lot at taco bell. A simple no works but its fucking rude. I'll usually do the kiss thing or say something like "I dont know, why dont we find out together?". Girls love a guy who's clean and well dressed and thats likely not why shes asking, for some reason youre giving off that gay vibe dude. or if youre not gonna try to mack to the next chick who asks you then ask her why she asked you, if nothing else use it as a conversation starter