I think I need to step back a moment and make sure that I am clear about what I'm thinking about abstinence as a means of preventing the spread of HIV. This will be my personal opinion, not that that doesn't show up here now and then anyway, but it's not endorsed by my Board of Directors (although I think they'd agree with most of it).
Abstinence is a fine thing. Children shouldn't be having sex. Period. They're too young to understand the physical and emotional consequences of early sex, and should be protected from both predators and the peer pressure that leads them to make premature decisions to become sexually active. Doing this is really difficult in a social and cultural context that continually confronts us all with messages that promote sexuality and sex. Parents should be supported in their efforts to protect their children from these influences, and we should all step back now and then to make sure that we are not part of the problem.
Abstinence for adults becomes a somewhat more difficult matter. Clearly, there comes a point where sex is desirable. The social and cultural assumption is that, with marriage comes entitlement to sex (which could lead to a whole other essay on sexual politics). There seems to be universal agreement among religions that procreation is a good thing, which pretty much means that sex to make babies is also a good thing. There is also a generally assumed right to privacy in our society, which says that people should mind their own business, butt out of mine, and let consenting adults do what consenting adults want to do.
How to reconcile these issues is part of the problem of the plain and simple, just-say-no message of abstinence. Of all the things that humans can do, you'd think that sex would be the simplest and easiest. In this age of ever present microbes, it's also one of the most dangerous. So people, at whatever age, do need something a little more sophisticated than "just say no" to get to the point where they can have healthy relationships, make healthy babies, and, more importantly for this discussion, prevent the spread of HIV and other STDs.
Abstinence as a concept is something of a tricky matter. Just like a condom, abstinence has to be used consistently and correctly. There's no "King's X" for prom night. There's no free pass just because you were drunk or high. To be abstinent requires the constant decision that says "no sex under any conditions." Then there's the question of "what is sex"? If Bill Clinton, bless his heart, couldn't figure that out, how do we expect kids to do it? STDs can be transmitted via all sorts of mucosal membranes, breaks in the skin, and other contact points. That leaves out quite a number of activities that young people are pursuing as "alternatives" to sex, but which still place them at risk for transmission of STDs. I'm not convinced "just say no," virginity pledges, and other such strategies quite get at the matter of "consistently and correctly."
Abstinence does, by the way, have a failure rate. Calculating it can be somewhat problematic, but we all know from experience that there are indeed factors that contribute to its failure. I mentioned peer pressure and substance use/abuse. Loooooove (cue violins) is a big one. Marriage and baby-making are, of course, indicators that, except for pledged celibates, abstinence isn't a plan for life anyway.
So--it's not a matter of whether children will be sexually active, it's when. If we can delay sexual activity as long as possible, that's fine. Let's all pitch in and work on that. But someday, the young'uns will reach a point where they will decide (or be forced) to become sexually active. What then? How do we make them ready for the risks?
Here's a hint: honesty, facts, reality.