Actually I think people acts according to circumstances. Kids are a blessing and a burden at the same time, always have been, and some other factors helps decide.
But unwanted pregnancy is not "a help to decide" and it's weird to say that the fate of civilization should rest in people being unable to have safe sex.
Low birth rates are only seen as a problem in the context of race or nation: "if we don't reproduce, our genes/our culture will disappear and foreigners will replace us". Globally there's no problem at all. When low birth rates extend to the whole planet, the problem will solve itself very quickly by elemental incentives.