Anti natalism right now is pro long term human survival.
There are too many humans on Earth. There are two ways to get the human population down to a point where we can sustainably live here. You can either exterminate most the existing humans or prevent new humans from existing in the first place. Which would you choose?
"Oh but that will trigger a demographic crisis when tbere's too many old people and too few young people!” OK? That's a temporary problem compared to the very very permanent problem of extinction. Which we're on track to doing of we keep living like this. And most of the issues of a demographic crisis has to do with recession and pensions, both unique to capitalism. The solution is to get rid of capitalism, not guilt people into having more kids to keep the capitalist machine alive.