We don't talk about birth control much because:

(1) the majority of CO2 emission is happening in North America, China, Europe, and industrial part of Asia, none of which exhibits high birth rate.

https://ourworldindata.org/grapher/annual-co-emissions-by-re...

(2) Birth control is already being tackled with in developing countries. It's just not being talked much in the context of global warming, because (1).

(3) Talking about population when the majority of CO2 is emitted by a small number of wealthy countries (with low birthrate) seems like focusing on the wrong thing. Especially since almost all arguing about it online are living in one of those wealthy countries. Yes, it would be really convenient for me if climate problem could be solved by telling Africans to get their shit together, but it's not happening that way.