The Western mindset regarding environmental sustainability is to encourage all the things circled in green (except eating a plant-based diet, for some reason). Note that Western civilization is the reason why basically all the 'problems' listed on the chart exist in the first place. (Further note that the person making the chart had to duplicate part of the chart to expand the bars in order to fill space and make it look like the other options even had any impact...)
https://en.wikipedia.org/wiki/File:Wynes_Nicholas_CO2_emissions_savings.svgNot having a child is an order of magnitude better than all the other 'solutions'
combined. However, this would break the cycle and there would be nothing to continue sustaining if children stopped being born, so Westerners do not even consider this as a potential option.
Of course, in reality, society would continue to exist and sustain itself even if families were limited to a single child (China has certainly not stopped existing). I guess the Western mindset is so heavily linked with infinite expansion that simply imagining all the potential grand-children, great-grandchildren, etc. that could not be born in the future is considered antithetical to the Western way of looking at sustainability. Therefore reducing the number of humans is off the table since they do not view that as a future worth sustaining?