I think the whole situation becomes associated with negative feelings when you get busted. A negative anchor is created in the girl's mind associated with making out with you. Her backwards rationalization tells her that "it was nothing more than a flirt anyway", and there may be some truth to that, depending on your game.
Depending on how superficial your relationship with the girl is, you could probably benefit by trying to bond with her a bit more, trying to connect with her at a deeper level and get to know her. That will make it "more than a flirt" so to speak, and you can always get back to her the day after and say "I'm sorry about yesterday, it was embarrassing... But it was still fun", confirming her feelings to build rapport again, and take it from there.
If you want to avoid these situations altogether, I would recommend that you interrupt the makeout before it goes too far. Tell her that "we need to take it easy", feeding the fire in her even more, and plant the idea of having sex with you in her mind - making her think and dream about it.
