In the modern context, if you impregnate someone without planning it out properly, there's a non-negligible chance they'll get an abortion, which is even worse for gene propagation. Furthermore, parents are to some extent legally responsible for their children's actions, so having too many poorly-regulated kids running around means exposing yourself to liability. A big part of the optimal strategy for present-day long-term reproductive success is to get rich, and a big part of getting rich is not having more kids than you can keep track of.

I think that's a retcon. People use contraception so they can have more sex than they would if they had to worry about having kids every time. They may or may not rationalise further, I suspect that generally they don't.