@N30-R3TR0 feels like author make woman pregnant for the sake of the plot. And as you said, there is no real reason to why they aren't pregnant unless they are impotent which is again impossible for such a convenient coincidence to normally exist. The only logical answer to this convenient plot is that they are using some drug or killing born kids (even if it wasn't mentioned). And if they even did control birth using herbal medicination (because they don't have access to pharmaceutical medecines), then no one of the womans would be able to work cause of the side effects (nausea, tiredness, organ failures like kidney), especially when they are doing sex every day.
Conclusion: Author just wanted to avoid the subject cause it's pain in the ass