Other legislations sound more reasonable than that: in France, the term for "Statutory rape" is "Détournement de mineur" ("underage corruption" or something). It applies to any adult (more than 18) that have sex with someone under 15. If the adult is in position of power (professor or something), any minor (less than 18) under his or her care is off-limits.

IANAL, but I believe the reasons it is a crime are the same: in the off-limits situations, the minor is believed to not being able of giving informed consent. But at least it's not called "rape".

(I find the term "Statutory rape" quite misleading. It is definitely not the same thing as plain rape, or worse, child rape. It looks like it has been pushed by a lobby of afraid, protective, conservative, possibly religious parents.)