This concept seems to elude a lot of people, but the literal breakdown of the phrase "statutory rape" yields "rape by statute" - which is to say, rape as determined by statute (not sexual violation by a legal document). The implication is that this is "rape in name only". If it was literally rape, then it would just be rape. But it's not. It's statutory rape. Being that the term "rape" refers to a sexual act conducted in violation of the victim's consent, statutory rape is applied in cases of consensual sex where one or more of the participants' consent is simply not recognized by the state - technically, making it rape. This is obviously not as bad as actual rape. Technical rape only exists so that we can tell certain people that they aren't allowed to have sex (for better or worse). It has nothing to do with what those people actually want.
Now, the inevitable fallout of claiming that "statutory rape is a consensual crime" is moral outrage. People will say, "but sex is inherently damaging to kids!" But we're not talking about "child sexual abuse" here. If statutory rape was child sexual abuse, then it would be called child sexual abuse (although there are zealots out there who would like to see this happen), and there would be no statutory rape anymore. But the concept exists because it describes a distinct phenomenon - one where actual consent (as opposed to legal consent) is present. And what about these things called Romeo and Juliet exceptions? Kids of a certain age (i.e., the age of sexual maturity, not to be confused with the legal age of consent) are in some cases explicitly permitted to have sex. Their "dating" pool is just restricted to their own age range.
So let's not pretend that nature gave nubile teens raging hormones knowing that it would destroy their minds and bodies, as opposed to, you know, contributing to the propagation of the species. Statutory rape exists for two reasons - to give a youth's parents legal recourse to pressure and punish them and their peers on the subject of becoming sexually active (because we live in a very judgmental, moralist culture); and, hopefully more importantly, to protect them from the possibility of coercion, exploitation, and abuse (none of which is guaranteed as a matter of course) by more experienced authority figures. Does the concept of statutory rape serve an important role in society? Is its inclusion on the law books justified? Quite possibly. But please, when we talk about this, let's not forget the very real difference between legal consent and actual consent. Because, whatever your agenda may be, I agree with Kinsey et al., who once said "that the happiness of individual men, and the good of the total social organization, is [never] furthered by the perpetuation of ignorance."