As regards interest in the CP of girls, I don’t have any doubt that the application of corporal punishment to the opposite sex is a major male fantasy. One only has to look at the advertising imagery to see that. I am not qualified to say if being chastised is a major female fantasy, but certainly the advertising imagery has often assumed that. Buy these jeans and get your bottom spanked by a handsome cowboy.
Since, in the UK at least, whole generations of men had their first encounter with the corporal punishment of the opposite sex when they saw female classmates punished, it is hardly surprising that some retain a certain fixation in that area. I’m not convinced about your rarity value point. It may be rare now, but it certainly wasn’t when I was a lad!