Possibly one of the most annoying fanon clichés I've ever seen:
Ranma is utterly naive and innocent in all matters sexual.
This is really annoying, especially because any author that uses this one usually takes it to the extreme - he has no idea what sex even is, and he's never even seen a naked girl before coming to Nerima. I've even seen a few fics that take this cliché to it's absolute limit - he's literally incapable of being sexually aroused, due to a brain injury, mental trauma, or Genma secretly using lust-blocking pressure points on him (the last one baffled me when I read it. Genma wants Ranma to marry Akane! How the hell would making him impotent help that goal?).
This cliché is complete bullshit. Ranma has gone to an all-boys Jr. High School, and he attends Furinken High. How can anyone seriously think that he's never attended a sex-ed class, or seen a nudie-mag or overheard perverted conversations in the boy's locker room?
His two best friends at school are Hiroshi and Daisuke, two notorious perverts. He's had at least one
perverted conversation with them. He doesn't look embarrassed or flustered during their interrogation, only angry that they think he would do those things with a "dorky chick" like Akane.
Besides those facts, he's a sixteen year old boy that can
turn into a girl. Do you seriously think he's never, ahem, "experimented"? Ranma is probably more knowledgeable about sex than any other boy his age.
Also, why exactly would someone who didn't understand sex be so skilled at seduction? In both forms, even! He turned on the charm for Shampoo during the reversal jewel arc, and later actually trained her mirror clone in how to pick up guys!
And he has gotten aroused before. Even in his girl-form, she certainly
got a kick out of watching dozens of naked girls bathe when she was trying to get to Happosai's Fire Burst scroll. And when Shampoo burst out of his bathtub at the end of her introduction arc, he fainted from "over-stimulation".
Sure, Ranma's almost certainly sexually inexperienced. He knows several girls who would be willing, but each of them come with a "sleep-with-me-and-you-marry-me" string attached. But he's sure as hell not innocent, even if he's still a virgin.