Or people simply don't know about it for sure yet even thou it's happening. Also, littered? Not really. Almost all monsters in the game rape by force not by magic. The goblins only give you drugged food after you are pregnant already, the parasites also have to infest you at a 4/5 rate before you lose your will to resist, only the cultists really open with the engraving of womb crest before the start fucking and then they go for full brainwashing.
But there is an even simpler and neater explanation to all this when looking at the lore of the other broodlings.
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The way I'm reading these is simply that the curse is not a 100% chance to happen, it just happens sometimes, and probably the closer we get to the advent of the otherworlder, the more corrupted the area becomes and the higher the chance is for babies to mutate. I would assume that the mutation chance has to do with ones resistance to evil forces and possibly how sinful of an act was that conceived the child. Again, I wouldn't be shocked if the pregnant woman committed incest with his brother and that is what's implied when she says that she shouldn't have done it and being an incest baby made it more susceptible to mutate.
On the same token my theory would be that the pigman for example was born out of legit bestiality and the sinfulness of it cause a "miracle" child to be conceived, and in my post above, Lilia herself conceives Pigman when impregnated by a pig.