This question was posed some years ago by political scientist Thomas Frank. I think Campus Progress just answered: sexual frustration.
There are so many things wrong with this law that I don't know where to begin. First, criminalizing any consensual sexual act is incredibly moronic. Second, presuming that all sexual contact between teenagers is "abuse" is a ridiculous premise. I know I wouldn't have considered it "abusive." Third, there's that whole right to privacy thing. Fourth, and most inanely, the dumbass pushing the law says that it's there to "help" sexually active teenagers.
You want to help horny young people? Teach them protection methods. But for the love of God, let young people decide for themselves how to handle their own sexuality. Big Brother shouldn't be in anyone's bedroom, Little Brother (and Sister) included.