Even if you are not “antinatalist”, what do you think about what I wanna call a “reducenatalism” (REE-dyoo-see-nay-tuhl-izm)?
Reducenatalism, like reducetarianism, seeks to reduce the number of children one will have. One does not need to be childfree/childless or antinatalist to be a reducenatalist. One needs only reduce the number of children one will have to below certain point. (e.g. below the fertility rate of your country) That would mean 0-1 child in rich countries, 0-2 child in developing countries, or any number below the fertility rate of your country.
Environmental or overpopulation reasons for having only one child was defended by Sarah Conly at the book One Child (OUP, https://global.oup.com/academic/product/one-child-9780190203436)
One of the excellant non-overpopulation reasons for the reducenatalism is that the amount of resources you can provide for your child per capita is divided “1/n” when you have 2nd or subsequent children. Consequently, when you are having 2nd or subsequent children, you are effectively “harming” developmental/educational prospects or even health prospects of your earlier/existing child(ren). If you have only one child and relatively well-off, you might be able to afford to send your child to private schools, hire a tutor, travel foreign countries with your child, buy (a lot of) books for him/her, send s/he to study abroad, send s/he to college and/or graduate school. Even in rich countries, there is a life expectancy difference between socioeconomic strata. (http://www.huffingtonpost.com.au/2017/01/31/low-socioeconomic-status-affects-life-expectancy-more-than-obesi_a_21704449/)
However, it seems most parents who have 2nd or subsequent child do rarely, if ever, think about the “1/n” effects of having another child. Of course, a child’s younger sibling(s) can provide some emotional benefits to the child, and the emotional or other benefits to the adult s/he will become. However, it is also true not just financial resources, but one’s time and attention to the child will also be divided as “1/n” when one has 2 or more child.