The first thing we need to do is recognize that a 13-15yo is capable of being a voluntary partner in sex. They don't recognize all the implications and their consent may be ill-informed, but we're talking about minors who are past puberty here... like it or not they are not quite "children" per se anymore, they are capable of reproduction and possess a desire for sex.
As the father of a 14yo, I hate to admit that, but it is a fact.
I'm not saying it should be "okay" for a 21yo to have sex with a 15yo. I'm just saying that if the 15yo "consented" then we are not REMOTELY in the same class of situation as the perv who manipulates and 11yo into sex, let alone any kind of forcible rape.
21+15... wrong? Yes, I'd say so. Life without parole, chemical castration, execution? Oh hail no. I'm not sure I'd even call it a felony as long as the 15yo was sober.
The actual predators are something else entirely... make the distinction, change the definitions, whatever... then quit giving the actual predators who prey on actual children NO second chances.