That's why you provide them meaningful sexual education,
then tell them not to have sex and explain why (if they haven't figured it out already from the lessons).
In general, you're confusing two things: willing abstinence and ignorance about sex. Indeed, a hypothetical "proper" Christian would, after getting the whole thing explained, come to the conclusion that abstinence is the best (from religious standpoint) option. Spreading ignorance has an effect of enforcing abstinence, up to a point where ignorant people "re-discover" sex, then it breaks down. On the contrary, proper sexual education would cause the need for birth control to become immediately obvious. For someone who follows Christian rules, abstinence is the only choice of birth control. In fact, I think that once it's not a big, forbidden unknown, resisting the urges would be a lot easier. Of course, this is all assuming an intelligent person who is also a devout Christian. For unintelligent ones, no amount of education will ever work (you'll always get a few idiots who just won't listen, whether it's sex ed, driving lessons or mandatory first aid training), and a non-devout Christian would just use a condom.
JP2 didn't personally claim these things, IIRC. People under his employ did, and he certainly wasn't disapproving those messages. On the contrary, his stance on the topic of safe sex was definitely not helping to contain the spread of HIV and other STDs throughout Africa.
So this is what I meant, and the clarification I asked for. It can't be said those were
his lies, though he might have believed them (as evidenced by him not disapproving of them). In general, sexual matters might be the area in which the Church has the most to learn. JPII could have definitely done a lot more to modernize the Church in this area, but at the same time we should not expect one man to fix everything.
I think that despite places where he failed, he still done enough to deserve Sainthood. Also remember that openness about sexuality and STDs is mostly a modern idea, and most of his pontificate was in times before it became as common as we take for granted. We should remember that he died about a decade ago. Maybe now that times are different and Pope Francis is at the helm, we'll see some advancement in that area.
EDIT: Talk about ninja'ed... Well, looks like I'm agreeing with InsaneBaron again.

Anyway, whether sex ed is done at all and how highly depends on the clergy in the region. "The Church" has been teaching people about
why abstinence is a good idea in some regions, where in others it just told people "if you have extramarital sex you'll go to hell". This is because it doesn't seem to be clearly regulated from the top, it's mostly up to individual priests, and they're a diverse lot.