Not everyone does that, and I think that's interesting. Some people who get turned on by the idea of spanking have no interest in anal whatsoever. And still others like the idea of anal but are not interested at all in trying it for themselves. (Or maybe they just haven't been with a partner they can trust).
The Church of the Illuminated series was a little bit scary for me to write because it was the first time I had characters really participate in anal sex as punishment or discipline. I have had other characters, like Jack and Meggie, have some pretty rough anal encounters, but those situations took place under a previous agreement between the two characters that this type of thing was going to be part of their relationship.
I felt safe in writing on this topic in the context of this crazy, fictional church because it was so far removed from reality. Even that has made some readers uncomfortable. Taking that anal discipline to another level, presenting it as semi non-consensual between two contemporary adults in the modern world would be pushing the boundaries. Wouldn't it?
