I think the law sees things very black or white, right or wrong. There's no place for the individual. How do you write a law for something as subjective as she's "a slut"? What qualifies someone as a slut? You might say someone is a slut, I might not. Thats where the age comes in, thats quantitative and not a matter of opinion. What's the difference between someone that likes to get some and a slut? Whether these girls wanted it or not Oscar Ponce was in the wrong. How much in the wrong is for the courts to decide.
I think willing or not when it's someone as young as 15 and you're 34 you know you shouldn't be doing it, you don't need the law to tell you that, at least you shouldn't!