I don't write smut. I write straight up erotica. I just don't write it here. This is the place where I put my more "wholesome" works.
IMO, I think every writer should write some smut at some point. Sex is part of the human experience, and it is a far more central aspect of it than anyone cares to admit. So, when you try to completely ignore this fact in your writing, it actually hampers your ability to write believable characters.
If you manage to keep in mind that, deep down, the entire reason any of us are alive today is because your mom and dad wanted to have sex with each other and thus it's a part of who you are as well as a human being, and the same is true for everyone else, it will greatly improve your writing. Every single person in your story should have some relationship with sex, even in completely non-erotic works with no smut in it at all. The works that have zero sexual content are just stories where people's sense of propriety beats out their carnal desires during the periods of time they are on scene.
(Also, yes, I really do mean every single character has some relationship with sex.)
Under-age children are unaware of sex. This counts as a relationship with it, even though it's far more distant than others in the story.
Pubescent-age children are trying to figure out the changes that are happening in their bodies, and the new feelings they're experiencing.
Everyone else who's in the more acceptable department for the subject of sex has varying levels of civility that mask their sexual desires, and varying levels of sex drive to go along with that.
So, yeah. Only the pre-pubescent children have zero actual sexual desire. Everyone else is masking it to varying degrees of success. (Some of which are very successful.) If you can keep this in mind and portray it accurately, you will write far more believable characters.