"Waiting until marriage" was around 4 years after leaving your parents house back when the average marrying age was 22 (assuming you're out on your own by 18). Now that the average is nearly 27, that's nearly a decade of restraining yourself on nothing but your own discipline and conviction.

It just isn't practical to most people who aren't pursuing a life in the church. And the people that do hold out for that long will likely (IMO) develop issues around sex that will take years to undo.