> Only a barrier method can be completely effective (and even that fails).

I would not call it completely effective. Someone who uses a condom perfectly, each time, every time, still has a 50% lifetime (35 year) chance of having a child.

And with more typical use a person is all but guaranteed to have at least one child during their life.

Condoms help reduce fertility (i.e. smaller family size), they do not eliminate the possibility of children.

Someone who absolutely does not want children does not really have any good solutions except avoidance. The typical advice is two methods of birth control at once (i.e. hormone plus barrier), but that's only an option for females.