While my youth was many years ago, when I was in high school and college, some kids had sex, some didn't, and as the kids got older and moved from high school to college, the number of virgins dropped precipitously. As far as it goes, it's true that if kids are really determined to have a bit of the old in-out (as Alex put it in A Clockwork Orange, they will find a way.
Where you're wrong, I think, is in thinking you can have no influence over whether they want to have sex or not, how much they want to, and with whom.
By raising kids with sound moral values, who know they are loved, and have real self-respect, you reduce the likelihood that your kids will have sex because they don't feel good about themselves and want someone to love them. You will reduce the likelihood they will have sex because they don't have anything else to do if they're active in school, athletics, music, church or any combination of the above.
None of these things will guarantee you kids won't have sex before marriage. If you're really lucky, your kids will wait until marriage. If your just pretty lucky, they'll wait until they're in a real relationship with another young person who respects them and isn't using them.
In any event, not to try is to increase the likelihood they'll be used by someone who regards them as simply a piece of meat.