To preface this, I have been writing since I was about seven years old. My sisters introduced me to fan fiction and I was immediately hooked. In my 13 years of writing experience, I always thought I would write a book and be published young. It was honestly my dream. It is always harder than you think.
One thing I never knew I needed to know about was sex. Now, this isn't for everyone, as I know plenty of people choose to not write about this mature content, but by the age of thirteen I knew if I wanted views and I wanted likes, then I was going to have to sell out. Thus I started my research as all teens do.
Even from reading and the occasional hentai I still knew next to nothing about sex. I had this vivid image in my mind of how glorious and amazing it would be, and deep down I knew the truth. Sex wasn't going to be amazing. So, I went to friends for personal accounts because I myself was not sexually active (a combination of my girlfriend at the time living far away and personal choice).
Sex blogs, toy reviews, friend accounts all gave me a good idea of what sex could be and what it was like, so I started writing. To this day, I only have three smut stories or stories with sex scenes in them. No surprise, they have the most views. But in my comments I am still critiqued on how I write my sex scenes and how the characters interact. I didn't know when I was sixteen what it would feel like, smell like, the interactions of two people having sex and how they feel. My characters were 2D.
I never knew at 16 that during sex you could find yourself laughing about Ghostbusters. Or that oral isn't always the most amazing thing in the universe. I have learned how to make sex scenes real, proper ways to stay clean. There needs to be actions before the act. They need rapport, even if it is just a one-time hookup
I didn't think I would need this information because I could just write teen fics and fan fiction, but you learn more about the human body. You learn how it feels, how it moves. Sounds, textures, sights, smells, taste. These are all important in writing, especially if you are writing smut. Once you have those down you have created the scene.
Once you have the scene, what sells it is the personality. Each character is going to react differently to their sex partner. Don't be afraid to have a character who absolutely loves it and enjoys it. Let them have fun. But they can also be indifferent. Knowing the feelings of the touches and everything helps decide how a character is going to act and react to those touches.
When I was young I never thought sex was going to help me build my characters and how they act. I never knew I was going to write about it, and yet here I am, more experienced in my writing, still unpublished, and yet I know I still have a lot to learn. When I was young I was naive to character building and scene building. Everything was flat. Yeah, not everybody wants to sell out to sex, but personally I enjoy writing about it. It has helped me grow, and honestly I am just as thirsty as others and I enjoy reading about it it. Sex sells, thus why I decided to make my first part of things I didn't know about sex.
There are plenty of more things I didn't know in my journey to become a better and published writer. But as we grow older, we live and we learn and I understand so much better why there are few young writers. They simply are just not experienced enough to write characters that are relatable to a wider audience.