Even not abstaining is not 100% effective -
Sorry, jeremy, 'abstaining', by the very definition of the word, is 100% effective.
I don't deny that failure to resist means that many who set out to abstain ultimately fail, but that doesn't mean that abstinence is any less effective than anything else.
... it may well be the worst method of not having children, because people do not want to not have sex and even if they do want to abstain, they are frequently incapable of resisting.
That, of course, assumes that everyone does 'not want to not have sex', and I doubt whether there is any evidence to support such an assertion. A good example is to ask the lads in a school about their sexual experiences. Having listened to their claims, a quick general knowledge test regarding sexual matters can show that anything up to half the lads have lied. (I remember being involved in such an experiment in one of my first jobs - when the local health authority tried to gauge the level of sexual activity amongst the 13-18-year old boys in the local schools.)
To quote yourself:
That's just expanding on the flaw I spotted. Still an idea only needs one flaw to be unworkable.
Your post had two such flaws.