Would you want your child to marry their high school sweet heart?
I personally would not want them to, or at least not without having a taste of adulthood.
One think that lots of teens in particular don't see is that there is a whole world out there outside of high school. The people who you share your life with every single day at school will drift away somewhat when you graduate. You will meet so many new people in college and in your career.
Do I want them to date or sleep with lots of people? Of course not, I hoipe they find love and someone to spend their life with. I also hope they live a little before they settle down.