Death sentence

Relations between adults and teenagers has been going on since the beginning of time and so far humanity has survived.

I'm pretty sure "humanity" would have survived with quite a few flaws as well. We didn't have to get rid of slavery either.

People being with teenagers is common in every culture, and in the animal kingdom.

In every culture might be a defense, but "in the animal kingdom" certainly isn't. Just because animals do it, doesn't mean it's right for us.

What would you think would happen if heterosexual relationships were suddenly considered illegal? Were you are considered immoral, sick, a monster for thinking about the opposite sex.

This is irrelevant. It would be relevant in a discussion about banning homosexual marriages, but you are comparing apples to oranges. Age is different than gender: gender has nothing to do with the state of mental development that you are in, while age clearly does.