Dren, you're viewing it from a perspective of intelligent, careful teenagers (In these cases, at least.). Most often, those who are that careful and wise in the matter are in an intimate relationship with their partner already - This, however, is very rarely the case.
Generally kids who have underaged love simply don't do it in order to reach a new stage of intimacy, but rather for the basic pleasure of it. And they aren't willing to take so many precautions for several moments of pleasure or for a person they don't plan to be with for more than a year.
