Comprehensive sex education gives the teens the power of knowledge to be safe if having sexual intercourse is something they want to do. Parents need to do their best in raising their children with good morals, and then help to guide them through their sexuality in their late adolescent years. The average age for sexual activity is 16; unfortunately premature sexual activity is happening, and there is little we can do to stop it. Giving the kids the information to help protect them is our best bet.
No, abstinence only sex education is not realistic. Most teens will have some sort of physical (sexual in nature) contact. It may not necessarily be sex, but it's important for them to be educated on the aspects of safe sex. Some teens will remain abstinent, but in my opinion those are the minority. Our society is over-sexed and constant reminders are everywhere.
I am not debating that abstinence is the best practice when it comes to pregnancy and disease prevention, however, teaching only about abstinence is a mistake. It is unfair to expect people to make informed decisions when they are only given part of the information. If people are not taught about actions and consequences, they are left to their own imagination and experimentation in order to learn. If this is the method we use, we can only blame ourselves when it fails.