So if society has a surplus of babies in need of care and a couple chooses to be celibate and adopt them they would be sinful of Onanism?

No. I am not a literalist. There are few bright lines in morality. I’m not about to draw one in this area. Absent special circumstances, choosing not to participate in the continuity of the society is, to me, a narcissistic and immoral act. Adopting orphans satisfies that demand for me. I am addressing those people who are simply too cool to be bothered with children.

“To have coitus other than to procreate children is to do injury by nature” — clement of alexandria

This notion has always struck me as a ham-handed way of getting Catholics to have too many children. I am a big fan of sex for fun and of birth control as a practice. Choosing not to have children ever is a wholly different thing from choosing not to risk pregnancy always. I take Malthus as seriously as the Bible.