It’s possible to write a character that is not experienced with sex without writing them as a child. I’m tired of seeing a grown man that is obviously level-headed, brave, and at least experiences attraction to others written like a child. Not growing up in a sex-crazed culture does not make someone childlike.
It can manifest in gentler ways.
Perhaps not having vocabulary for body parts
Viewing sex as purely related to reproduction
Shame when feeling sexual attraction (if that’s been cultivated in the culture they come from)
Wanting physical contact that may not be considered inherently sexual (stroking hair, hand-holding, back rub)
Being awkward during the act (Face slamming into someone when going in for a kiss because -fuck- he didn’t gauge momentum and distance correctly at all)
Not knowing literally any sex-related slang. (Dick? Make love?)
Spending too early if he’s had to spend his life hiding the fact that he masturbates.
Never fucking cumming ever because he’s literally just waited boners out instead of taking care of them and his resistance to orgasming is off the charts. (Finn, come on dude. My jaw hurts.)
Not having any sort of sexual reaction to seeing people naked because he didn’t grow up in a culture that sexualized nudity
Finding weird things sexy like Poe saying “buddy” because why not
He doesn’t have to be an idiot in fic. Not knowing how to fuck doesn’t mean he doesn’t know how to talk like an adult. I think, especially in the US, we’re so over-sexed that we think sexual knowledge = adulthood. It really doesn’t.