If you initiate anal you are likely to be knotted within two turns, which results in an end to battle and potentially a game over.
If you look at the side menu during battle that lists potential endings, there is one to be knotted orally as well, and iirc, it's supposed to be an automatic game over.
I know how to get oral started, and she even swells up like the knotting anal position, but it never seems to initiate the knotting oral sequence.