First, because they lack of awareness and information. There is no to orient them with these things and correct their metaphor of love as engaging to sex. Second, peer pressure. As time flies, the teens learned to value liberty too much that they are seeing it in a wrong concept. So when they happen to have friends who are wild enough to engage in unprotected sex and they are the only one who haven't done it yet so there comes this feeling that doing it too is mandatory. Third, I don't know. Maybe they were oriented, maybe they were aware but the thing is they got so curios to the point that they ignored all the precautions given to them.