Regardless of your personal feelings or moral beliefs about sex, it’s natural for the subject to come up in your fiction. Nearly every human does it, with varying degrees of frequency, adventurousness, guilt, and pleasure. Sex scenes are a staple of romance fiction, mandatory for erotica, and the only reason to create pornography.
But even that doesn’t cover the breadth of fictional sex. Literary and crime fiction have plenty of it, horror stories are strewn with the bodies of lovers who sneak away from summer camp for a tryst, and in this writer’s opinion, there’s no reason to avoid sex in science fiction and fantasy either. Of course, it’s a matter of taste, the writer’s and the audience’s. A scene appropriate for erotica will probably feel out of place in your cozy mystery or humorous fantasy. The type of coarse scene that works for your gritty private investigator isn’t right for a romance story. Your average sci-fi story won’t benefit from a description of your literary protagonist’s unique masturbatory methods.
But that said, we all do it.
If it’s appropriate for your current work, consider how your protagonist thinks and feels about sex. Start with their childhood – what did your hero know or hear about sex when they were a kid? Were their parents openly affectionate and did they have “the talk”? Or was sex a subject of embarrassment or shame? Did they learn about sex from a trusted adult or from books? Did someone give them a book or did they sneak to the adult section at the library? Did they obtain their knowledge from a gym class session on hygiene, an actual sex education program, or from the playground expert? Did they ask questions? What answers did they get? Who did they ask and how did that person react?
Whatever happened, examine how this formative experience stuck with your protagonist through the years and consider which parts of their sexual upbringing are still with them today.