At some point they did say that natural insemination rates in that world were really low.
i could've sworn that was another series with similar premise, not this one. can you remember which chapter had that info?
So nothing in these R18 chapters, and re-reading all of the regular timeline chapters there's nothing about conception/insemination rates are low in this world. But the rate can be inferred.
IRL males tend to be conceived at slightly higher rates than females, something like 105:100. But there's various factors such as a more vulnerable immune system and men being active in conflicts, that end up making it so that in most societies it's a 1:1.
So something had to have happened in this alternate world where we went from 105:100 to 1:39. Something on a genetic level, had damaged the reproductive system of humans in this story. And given how often Sou creampies the women in his life, but have not yet been impregnated, there is a serious problem with how often children are even conceived (hentai logic aside).
Edit: The behavior of the women being all "GET ME PREGNANT PLAPPLAP PLEASE~" can be justified as a necessary adaptation not only due to the rarity of men, but how hard it is to get pregnant in this story.
Edit2: writing out 105:100 next to 1:39 is a bit of fridge horror. That's a near extinction ratio.