Everyone is not addressing the main reason behind these laws is to protect kids from getting into a fucked up situation if they get pregnant. 99% of the time they are not married so you have a major legal issue at hand that leads into the abortion issue. And we all know how divided the world is on that. Also if you get someone pregnant and they decide to keep it, you've now got a guy that has to support a child and basically has fucked up his life since most relationships formed in your teens are not what you want when you are an adult, you've fucked up the baby's life because odds are he'll grow up in a fucked up parenting scene, or thrown into an adopted home or into the governments child protection agencies, and a girl who has to endure a pregnancy and all that entails psychologically for 9 months and messes up her life pretty much before she has experienced the real world.
That's the bottom line here. I don't think its so much about sex, because yes what's really the difference from 16 to 18? Its more trying to edge it up so that at least the maturity and responsibility levels will at least be a little higher than if you have 12 and 13 year olds fucking. Most teens do not use rubbers when they do their first fucking remember
, so the chances of getting knocked up are very very high.