If we're going to venture into logics, what you got there is an ill-formed statement. Children are not an (intentional) substitute for contraception, but a result of its absence.
If usefulness of toilet paper is not self-evident to you, I'm afraid our differences are too great to be settled by an interwebs debate.
Alright then, replace children with cigarettes - pretty sure they're used much more frequently than contraception in many parts of the world - doesn't make it more useful at all.
And saying that toilet paper's usefulness is self-evident is equal to saying "My dick is bigger than yours because I say so and it's so obvious I don't have to prove anything to you".
Wonder if that's ever worked to convince your opponent in a dispute.
Surely they just take your word for it and don't laugh at all.
Btw, talk about toilet paper to someone in India and you'll see it may not be equally useful to everyone afterall.
The usefulness of cigarettes and contraception could be compared at some level. Cigarettes are more useful to me right now, because I'm not in Africa & not planning to fuck there. If you wanted me to invest in African contraception, I'd pass. Because I'm not planning to fuck in Africa, hence the product is useless to me.
I'm also not buying shit drugs on lameass neckbeard scamsites, so I find Bitcoin equally useless.
I hope this clears things up.