Let's go way back in time for a hot sec and talk about the Reverend Thomas Malthus, who wrote "An Essay on the Principle of Population " in 1798. As his name suggests, he was the founder of Malthusianism, which sounds like it should be the opposite of "enthusiasm," but in fact is a philosophy that advocates population control.
You could say Malthus was not enthusiastic about population growth.
But why was that? Malthus believed population growth was exponential (think how many kittens one pair of stray cats can produce over five years), while food production was arithmetical (meaning it just got added, not multiplied by itself). This led him to the conclusion that eventually there would not be enough food for everyone, which would lead to bad stuff like war and famine and so on.
So, Malthus, how do we stop that from happening?
"Easy-peasy," Malthus answered. "Don't have so many kids."
And what's the best way to do that?
"Whatever you do," said Malthus, "don't use birth control. That's weird and icky. Delay marriage until you can afford to have kids, and also practice abstinence from sex within marriage to limit family size. Oh, and don't have sex outside of marriage. That goes without saying."
Right, Malthus. Because that will happen.
About eighty years later, here came the Neo-Malthusians. They said, essentially: "Hahaha, Malthus. Practice abstinence? That's hilarious! People definitely won't be doing that anytime soon. But seriously, Malthus, you're not wrong about this population problem."
The Neo-Malthusians accepted Malthus' basic theories about population growth and resource production, but they said it was okay to use birth control to prevent unchecked population growth. They also identified some groups of people they definitely don't want reproducing.
Top of the list? Poor people.
As the 20th century advanced, the Neo-Malthusians became more and more about birth control, but their concern with birth control was always, at its heart, a concern about population growth. But where did birth control fall on the political spectrum? That's not as clear cut as you might think.
See, the Neo-Malthusians concern was never really about women having the right to control if and when they had children, a typically left-wing concern. Nope, they were way more concerned with making sure the poor didn't get out of hand, which made them natural allies of the typically right-wing elites.
At first, governments were against birth control because that cannon fodder doesn't just make itself, you know? But gradually, as they realized their populations were getting overrun by poor people, refugees, and immigrants, they came around. The same goes for many religious organizations.
In a nutshell, the Neo-Malthusians were all about the birth control…but only as a means to stop unchecked population growth.