If Your really squeamish I don't what to go there but probably will so read at your own risk.
I am going to explain this in two ways issues should all ways be looked at; emotional, gut or a humanistic approach and a logical, reasoned or by the numbers approach.
On the logical front we seem to be missing good numbers. Ever time i hear numbers it seems to be from Now, Planed Parenthood or the Christan values whatever group. I haven't looked for the numbers but if there where good numbers some one might have said something. If some solid numbers where to surface and it showed 50-60% or more drop in STDs and pregnancy especially in problem areas you might be able to convince me.
So what we do know. Abstinence, 100% no Pregnancy and no STDs. It is however not the only 100% there is masturbation, mutual masturbation(RIP Joycelyn Elders political career), and any of the less sexual alternatives the strict Christian Conservatives might proscribe (A lap dance maybe, sorry).
Condoms, the pill, dental damns, and female condoms (full body condoms) aren't 100% but do a pretty good job.
No protection is leads to STDs and pregnancy.
An unwanted pregnancy is not usually a good thing and there are far to many of them.
Young people have as a group less self control and are less educated and experienced in these matters physically and emotionally.
There is a strong biological and emotional and drive for sex.
The religious aspects are important but should not be the driving force behind policy.( How is having information about sex a sin? Not encouraging, it just having the information.)
To the second way to look at it and it is a much more opinionated one. My gut, what i really think the abstinence only crowd are completely freak'n insane. The Bible is not really a good contraceptive and wishing so doesn't make it so. I might deter some but not enough. A sex until marriage argument doesn't have to mean no information on sex until marriage. Fed funding for abstinence only it would be freak'n funny if it wasn't true. I will admit this comes from a more open view.
Is trying to resist the urge of having sex just a pretty major distraction when your young? Can't we find ways to relive that urge with in the things that are 100%? There is some area betwwen abstinace only and teaching prvention and teaching prevention isn't teaching The Karma Suta and S&M. None of this should be a step by step how to.
Abstinence only: 100% prevention if followed. No good alternative for sexual drive( A long walk with a close friend doesn't fill the same need, it might be just as fulfilling when your older and a little more emotionally developed not the same and not at 18)
Abstinence from physical contact sex: 100% prevention if followed. Alternative such as masturbation and mutual masturbation taught. Not Descriptively taught but offered as alternatives.
Physical contact sex: 100% for Pregnancy not necessarily on the SDTs if the are open wounds. Manual stimulation and other skin to genital contact offered as an alternative. Again not a how to lesson but the transmission of exactly how STDs are are spread should be covered.
Actual sex with prevention: Nearing the 100% with correct use of prevention. No Hoe to just the parts to get the prevention right
Let them figure it out: Not good and where you end up if top three aren't followed. Not really a teaching policy.
No matter how much you demonize sex with the truth or false facts and number you won't prevent as much with abstinence only as you will all the facts. Look information is power. Being informed doesn't mean you have to do it. Letting STD's spread and having unwanted pregnancy isn't an alternative.