It is a law. It defines statutory rape as unlawful sex, whether both parties are willing or not. There are legal ages for sex, like 16+ with others under 18 but over 16, or within 4 years of their age. Then anyone over 18 can only have sex with someone, I think. Give me a sec and I will post a link to a wiki of it.
Sexual intercourse with by an adult with someone underage
I think it's when the sex is not against the victim's will but the victim is a minor (under 18 years old) and the raper is 18 or older. To be honest I think it's dumb because I don't think every situation like that is rape. If it's a little kid and an adult then yes it's wrong but if it's a 14 year old and a 19 year old then I don't think it's wrong