The main reason for this is that teenagers are simply too impulsive and undisciplined to use condoms with the rigorous care needed to avoid failure. To make matters worse, supplying teenagers with condoms inevitably produces a marked increase in their sexual activity. On the contrary, all the available evidence shows that condom distribution will only worsen the consequences of teenagers' sexual activity. OK, so we see that there are a plethora of organizations that want to sexualize children. I certainly don't have an absolute answer, but I could make a guess.
Sex is a natural urge that is difficult or impossible to control.
The principle of freedom and personal autonomy demands that everyone has the right to have sex with anyone, at any time, at any age. You might guess that I put the most significant reason last. We can easily refute the first reason as being inconsistent with our religion. We can refute the second one by clearly distinguishing the difference between animals and humans, in that humans can control natural urges, otherwise there could never be any hope of a civil society. We can refute the third one by the simple statement that even in a free society, there must be rules for living, and especially rules to protect the innocent children.