> When I was little it was still married-with-children by default, probably at relatively young age, and people had to decide not to have children.
Don't ignore selection bias. When you're a kid, almost everyone you know is a child, and so almost every adult you know is a parent.
I don't know how old you are, but most of those societal changes - no need to marry and contraception - have been in place for decades. I'm in my early 30s and I wouldn't say staying single and not marrying is the "default" - yes, there's more people doing it, but I still know a lot of people who have coupled up and got married. I don't think I know anyone who's completely checked out of the idea of a relationship yet - the single people are still dating, often with the expressed desire of finding someone.
I have 2 kids, and the main reason people I talk to aren't considering a third is cost. Cost of an extra bedroom, cost of a larger car, incredibly high childcare costs - there's a very large real reduction in living standards that comes from going from 2 to 3. Compare that to my parents generation, where 3 or more siblings was the norm. I suspect a large part of falling birth rates isn't that loads more people are living child free lives, but that the people choosing to have kids are having fewer of them.