I don't know if porn really affects population growth. There are so many other factors that do that to a much greater extent, such as: 1) gender equality and women working, 2) education, 3) contraception, 4) lower child mortality, 5) the fact that 200 years ago kids were an asset (free labor on the farm and support in old age) but today they're just a huge expense, 6) the fact that even a few decades ago child care consumed much less time as kids could be left to play outside, but today you have to supervise them 24/7 at least until they're 12. Etc etc. people don't have kids for a long list of reasons, none of which has to do with men watching so much porn they can't have sex even once.

But even if that was true, who's to say that population has to keep growing? There's 8 billion of us, isn't that enough? Housing prices are through the roof and most young people can't even hope to be able to afford a house. Human population has doubled about 4 times in the last 100 years. If it doubles yet again there's going to be 16 billion of us. Do you think the world and humanity can sustain infinite exponential growth? I don't think so. The only reasons to want population growth is because the pension system is a Ponzi scheme, but that's a completely different problem.

I also find it interesting that on one hand the argument against porn is, "porn is bad because it encourages X behavior in real life", but then another is "porn is bad because it discourages sex in real life".