I've never kissed/made out with a girl before, but recently I've thought more and more about kissing:
It's totally normal. It's more likely to happen when you're inexperienced and highly excited by the novel new experience of making out.
It has definitely happened to me. It's not weird at all.
Thanks for the responses everyone! Now I have one less thing to worry about if it should happen one day. :)
I suffer the same problem as the op, but more severe. I sometimes get wood when merely talking to a girl, and it's really embarrassing in social situations, but I've become a master of hiding it...