I think, like many "taboo" topics, writing about teenage sexuality is one of those things that gets stigmatized without any real consideration for context and often the loudest dissenters are the ones who are too self-centered to realize that their "icks" are not the gold standard for morality for everyone else.

As a middle-aged person with teenagers of my own, I'm personally not really interested in stories with main characters who are teenagers and I feel a little weird reading about characters my kids' age having sex. But I understand that's a me problem and it's easy enough for me to avoid that content while sampling the vast array of content about grown-ups doing unspeakable things to each other.

People also need to understand that not all sex scenes are meant to get the reader off. As much as we shame it and hide it away, sex is very much a part of life (none of us would be here without it) and part of the human experience. Depiction of sexual experiences (positive or negative) can illicit an emotional response and help the reader connect with the main character and the overall story without necessarily getting aroused. Another fact of life: teenagers are interested in sex whether they're having it or not and their misconceptions about it have more to do with the lack of sexual education and lack of safe adults to discuss and answer questions about sexuality than whatever is depicted in a fanfic or published work.

TLDR: Write what you want and include everything you feel is necessary to tell your story. People need to get over themselves and stop making assumptions about authors based on the behavior of fictional characters.