The overwhelming majority of sixteen year old kids have clue why they do the stupid things they do and lack the maturity to understand the consequences of bad choices. Even then, it's not so much about protecting them from their own stupidity as it is keeping people from preying on that stupidity and protecting the rest of society. Most kids at that age are like sheep who can be easily manipulated by the promise of something illicit, be it alcohol, drugs, or sex.
A teacher, especially, has the potential to hold a high amount of influence and authority. As adults, they should understand that. There is no getting around that. They know what the rules are, they know the repercussions, so if they break them to have some meaningless sex then I have little sympathy.
