What are you talking about? FYI, there is absolutely no mentioning of this in the game except for that one line from the pregnant woman's brother that some priests were taking away pregnant women and their babies. This line simply serves to explain the villagers' mistrust of the church and a prelude to the corrupt priests that show up later in the game. There is no in-game evidence that the church itself was actively taking away every pregnant women or that "all fetuses were being corrupted by otherworlder influence."
I usually remain as a lurker here, but I'll just break my silence and throw my two cents on this topic:
For the 'pregnant women' one, you are correct that it doesn't confirm "all priests" partook in the act, but at the same time it doesn't really matter. The church had been taking away the town's pregnant women to the point that the townsfolk were suspicious of Lilia which implies at least an substantial amount of time this has been ongoing and that the priests were in on the plot. The fact that Lilia and Haanja are operating independently without reporting to a higher authority (like a priest) further adds to this.
There is no in-game evidence that the church itself was actively taking away every pregnant women or that "all fetuses were being corrupted by otherworlder influence."
Yeah, the pregnant woman's brother mentions this bit in their dialogue and he also points out how both Lilia and Hannja are different from them. But again, that only hints at the corrupt priests you run into later in the game. It doesn't really prove any sex results in abomination brooding.
As for "no proof that all fetuses were being corrupted", it's literally stated on the Abomination Broodling Harbinger gallery text:
"A being that exists between the mind and body of a pregnant woman, forming the origin of an Abomination Broodling.
When sperm reaches its destined place and merges with the mother's blood, a formless wickedness from beyond this world seeps in.
This intrusion congeals, eventually taking shape as an Abomination Broodling."
The text straight-up says that this is the case. It's why being impregnated by demi-humans, pigs, cultists, parasitized villagers, or even regular townsfolk has Lilia getting pregnant with the same being. The Otherworlder's influence has gotten to the point that normal births are impossible and all fetuses in the womb are being converted into becoming its Broodlings.
And if what you are saying is true, the church should have sent out a message prohibiting any form of intercourse during their purge of abomination brooding. There is no mention of that either.
Again, if any pregnancy results in abomination brooding due to corruption in that region and everyone knew about it, there is no reason the pregnant woman's brother to be surprised about his sister's abnormal child or Lilia suspect the woman of being pregnant with abomination.
That's because no one has the complete picture; not the villagers, not Lilia, nor Haanja. And if they do know, they're in the cult trying to facilitate said births in the first place. The local church didn't say anything because the cultists had great sway over it and doing so would harm their operations, (The villagers wouldn't give up their pregnant women to the priests if they were informed after all.) and those outside the cult only had limited information. Lilia and Haanja were more well-informed about the births, but unlike the player, they weren't exactly sure if all births were guaranteed to become Abomination Broodlings or if regular babies can still be born, just with a very small chance of it actually occurring.