You are misinterpreting the intent of the law. The idea here is to protect children under a certain age who are incapable of making adult decisions for themselves. Feel free to have sex with my 15 year old. Just ask for my permission first.
So, everyone who has had sex before the age of 18 was "incapable" of making that decision? Give me a break.
Look, people can debate around the clock whether teens having sex is a good idea or not. But, one thing I'm quite sure of is an 18-year-old & 15-year-old having sex should not be a criminal act. That is Big Government stupidity.