For one, just because it's a law, that doesn't make it right .... abortion is both legal and illegal, depending on the country. Same for the death penalty ... Even statutory rape has different requirements in different countries (or even states).
Also, not all offenses punishable under the law are serious ... If you forget your driver license at home and you get caught, the law says you are guilty of a "crime". There are many level of "guilt", and the way I see it, "statutory rape" is right there at the bottom, with the speeding tickets and parking fines. The word "rape" has absolutely no logical meaning in that expression.