First of all, this character is a she. There are nearly no exceptions. She lives in middle-class America (or its equivalent), and she’s the extreme version of the girl next door. She knows nothing about sex, and if she’s ever kissed someone, it was a sweet peck and nothing more. When the people around her make sex jokes, she’s completely lost or maybe only has a vague inkling of what they’re talking about. She’s never bothered to google anything, and the extent of her knowledge is anatomical and medical at best.
Why this is bad: Sure, there are people in the world who don’t know anything about sex, but the chances of those people being matched up with this background are slim to none. Have you seen public school? Or private school, for that matter? Kids talk, and even the most virginal of people learn a lot from their friends. The odds that this girl knows nothing is about one in none. Whether it’s kids being curious about their own (or others’) bodies or a poor choice of google searching, they figure these things out on their own. And once you’ve hit 23, you’ve at least learned some stuff secondhand. Even asexual people are not excluded from this, as many have either had sexual encounters before or have heard the same chatter around the lunchroom table as everyone else.
But where this trope gets especially gross is that these characters, after being shown to be naive about sex, are then highly sexualized and even infantilized. Not only does this give off some bad porn vibes, it boils the character down to this single trope. The character’s sexual awakening is either used for laughs (”Now that she got her first boyfriend, we keep finding them making out in the kitchen! Ha ha, young love!”) or to make fetishistic sex scenes (a la 50 Shades). Either way, nothing good comes of it.
How you can fix it: As always, I’m going to tell you to think of your character as an actual human being. Do you know people in your life who live up to this trope in some ways? I know I do, but those people still understand sex and dirty jokes. Just because they’ve never done anything themselves doesn’t mean they don’t get the innuendos. It doesn’t mean when they walk in on their roommate and her boyfriend snuggling naked underneath the covers that she doesn’t know what might have just happened. She’s had 23 years to learn this stuff. While these people might not know as much as others, they’re still going to have a basic grasp on what sex is and what it involves. And as for the sexualization and infantilization of these people? It’s gross. Stop.
Bottom Line: By this age, everyone’s going to have some inkling of sex and have heard some stuff. Keep it realistic, and don’t make it porny.