[...] I think the only genre where teen sex happens consistently and is presented in an unjudging way is the horror genre... too bad the slasher always comes and kills them in the middle of it, lmao.
Ignoring the question of how we got to this topic, teenagers having sex should not be presented in any way. It's one of those things where you should tell not show. It's really damn creepy how some media, including horror movies, will establish their characters as underage (or barely not underage) and then show the characters have sex. Especially considering that most sex scenes, regardless of age, serve no purpose and are just there to grab the viewer's attention.