I can't believe you asked "you wanna be my girlfriend?" You don't ask, after a few dates it just kind of happens.
Yeah, this sort of question is usually abandoned by the time someone reaches the end of high school. Typically you just let things progress, then if for some reason you still really need clarification you can have the "so where are we at in our relationship" conversation sometime further into the relationship when your communication has evolved beyond the 'Did I say the wrong thing?' 'Did I scare her away?' 'Does she still like me?' 'Should I call her or wait for her to call me?' phase.
You said the two of you kissed? How did that go? If she was going to have a negative reaction to that, it would have been immediate (or at least I'd think it would have been). If she let you kiss her, I can't imagine her being scared away by you asking her if she wanted to be your girl friend. But seriously, when you are to the point of touching lips, you're usually beyond the point of asking if she likes you or not. Unless she just wants to have a good time with you without really being in a complex relationship, which, if that is the case, is something I would highly recommend you grab hold of and fly with.
"Every time Warb sighs, an angel falls into a vat of mapel syrup." - Gaffer Tape