Not getting the connection here. I don't think contraceptives are used to prevent the existence of a child living a life of suffering. They seem most often used to avoid inconvenience on the side of the potential parents.

One could argue that billions of children have no chance to exist because of contraceptives, just as you argue that billions of animals have no chance to exist if we stopped breeding them.

Oh I see, if existence is worthwhile, even in the face of suffering, then contraceptives are preventing something worthwhile. Yes, the logic applies here too.

And, it raises an interesting question about the moral gravity of preventing existence.

If they don't exist they can't be harmed by not being brought into existence; since there is no one there to suffer a deprivation.