Aiko is an example that doesn't hold up if you think about it. She only says that she doesn't have sex with people who fail the temptation challenge if she doesn't like them, and if they come back again she lets them through without a fight. We know that 75% of challengers eventually clear the temple though, and presumably most of those people can resist her temptation challenge if they're capable of winning a sex battle against the Oni Twins. So even if she's letting some percentage clear her side of the temple without having sex with them, she's still getting dicked down on the regular.
That said though, I don't think there's a formal explanation for why none of the monster girls the player encounters ends up pregnant despite all the sex they're presumably having. The generic elves in the forest are very much trying to get pregnant in their encounters, even though nothing ever seems to come of it. It seems like Threshold thought better of calling attention to possible pregnancy after he wrote them, because a quick search through the code reveals that few other monster girls bring up pregnancy as a consequence of sex. Kaia compares the PC getting addicted to her venom as if she got them pregnant, and Minoni will reveal she started taking herbs to prevent pregnancy after the PC starts having sex with her, but that's it for the base game. I did add a mention of pregnancy to one of Beris's conversation options in Azure Blaze, but I dunno if any other mods reference the idea.
