Underaged Teens Having Intercourse

1) Teens under the age of consent having sex with someone over the age of consent. 

Any child old enough to be thinking of sex is at fault, whether it be the male of the female. Neither is more at fault than the other unless it is actual, rape. 

Another person at fault is the parents (in most situations) they should have been informing their child on the risks of having intercouse (pregnancy, stds, breaking the law, etc) Simply telling your child, "don't have sex" won't do shit. Believe me, I was there two years ago. Thankfully, I informed myself on all of the consequences and waited to have sex until I knew I was ready and I am now married to the man I lost my virginity to (not saying that's the right way to do it, it's just how I did.) Some people aren't so lucky and try to find out things on their own then fall pregnant when they are clearly not ready. It's all about being open and honest with your children and giving them every ounce of information you have on the topic to make them realize it is when you want to have sex. 

2) Telling your children "sex is bad." Sex is not a bad thing nor should it be frowned upon. If you inform your children enough on the topic and they wait until they are ready, I see no problem with having sex before they are 18 or not waiting until they are married. If you waited until marriage, wonderful. But that's not for everyone and not all people want that. Sex is a beautiful, wonderful thing that you can share with someone. Whether you want to do it smith someone you love or just do it to do it, as long as that person is happy with their decision and is being safe, them having sex is NOT bad. Telling your children that it is a bad thing to do is wrong. That just teaches them to worry and think they are a terrible person for doing it. Sex is a natural and awesome thing as long as you are well informed on the topic. 

Let's be real here, once your child is in a relationship and starts to think about having sex, nothing you do or say is going to stop them. They WILL do it if they want to. It's better to prepare them and accept it to keep a good relationship with your child then try to fight it. 

UPDATE: I'm not speaking for extreme relationships such as "13 and 45" as stated in a comment below. More like realistic ones such as "15 and 19" and ones similar to this.