After you get the ability to visit Kasyrra in her home, it's she suppose to make the player give birth to demons if Kasyrra fucks the player? It's that what demons are suppose to do?
I believe it happens every time, after my first scene with her my champ got imps because I didn't know that. After that, I used the brown leaf to make her infertile so it wouldn't happen again. And yes, is something beyond Kas control as she explains, she will always have you or anyone, herself included only breeding imps.
Probably not written to happen honestly, probably to get the point across she fucking hate imps, and doesnt want to ruin you with them. Considering your in her hell house she has told you how she wants you to give her a real child.