I don't think anyone is saying no one ever makes that choice, just that it isn't one that most people will make.Even back in the day when there wasn't available birth control and a very strong social pressure to not have a kid out of wedlock many couples did not wait for their wedding night.
There is a reason my Mom grew up with the saying "an eager maid can do in 6 months what takes a cow or a matron nine". Now I think more couples waited until they were likely to get married (or ended up getting married anyway), but there used to be a heck of a lot of 7-8 pound "preemies".
Do people wait, sure, but assuming that a lecture in a classroom is all it will take to make all people stop having all premarital sex, I dunno how well that will work (even if a lot of the programs try to make birth control sound too unreliable to work and too risky to even try).
So far the data of trying to keep kids out of each others pants doesn't seem to do much other than delay things a few months and/or make them less likely to use birth control when they do.
That being said, there are good reasons to be abstinent and I firmly believe it should be one of the options discussed in depth in any sex education course. After all 100% is still different from 99%, when assessing risk.