I'd like to see one where the MC was friend zoned and the childhood friend sees them as a brother.
They don't like seeing the MC getting any because it feels creepy to imagine their psudo-brother getting laid.
The male MC keeps guys away from the girl, like a protective big brother, but everyone thinks it is due to romantic feelings he had for her.
They don't realize they feel like this, and thus can't express it clearly, so everyone thinks they are just in denial.
Then again, this style of stories was created by the Japanese, and they seem to have no real taboo about family getting it on in stories (at least the ones I've seen. I don't know about real life)