Well it usually doesnt, but if it does then the girl must be super hot or you must be feeling on her or you just want to do her.
Listen homie if you're getting hard when you kiss a girl that because => you're a guy and you're NOT gay. When you're kissing a girl you're engaging in a sexual event that usually leads to sex. Which is why its apart of foreplay, so chill out and relax! Its all good! #Legit