Okay I've had a think on it and my serious answer is this: We can argue that the world has always been full of exploitation and suffering and pain and that there have been dark times when it seemed like the eschaton was truly at hand (there were people who saw the black plague and still had kids). There's two explanations for why:

1. People still wanted to fuck despite birth control being unreliable or unavailable.
2. Kids were an economic necessity - you needed the extra hands to make a farm productive, and the social expectation was that your kids would take care of you when you were old. It was a standard part of the retirement plan.

Re: 1: Now we have free porn and consequence free sex
Re: 2: Now we have 401k's and - pfft, who am I kidding, no millennial thinks they're going to be able to retire and the atomization of society means we're currently relegating our own parents to nursing homes and who wants that? If I'm still alive when I'm 80 I'm going to wander out in the woods and take mind-dissolving quantities of hallucinogens until I get to go face-to-whatever with the eternal Dao and ask it what the hell it was thinking.

tl;dr - having kids used to be unavoidable or necessary for your own survival, and now they're not