Those are almost always No True Scotsman fallacies. Supply-side failed because it wasn't supply-side enough. Etc.
Practiced perfectly, yes, abstinence is 100% effective at preventing pregnancy. There is no question about that.
And yes, some small fraction of people do manage to practice complete abstinence, and reap the benefit of not getting pregnant.
But as a policy, abstinence only for broad swaths of the population is a total disaster.
> some small fraction of people do manage to practice complete abstinence
I actually think it's a pretty large fraction, but they do it out of inability to secure a mate. The people for whom supply and demand curves never cross.
It's much harder to reason why a person capable of having a life will choose to not to.