Pledging Purity?

Why are all these young singers pledging purity by saying they will not have sex until they are married?  Who are they kidding?  I don't think you need to wait until you are married to have sex.  Actually, if you do that, you will most likely have sex once and then never again until you get a divorce.  Why can't these teens just wait until they are 18...then have sex.  They don't have to have sex when they are 15.  Just wait a few years.  Then do it.