...You know, I have no idea if consensual, loving, procreative (but not part of a fertility spell) missionary sex within a Catholic marriage in an empty church sanctuary would be okay or not! The question has never come up, and it's surprisingly hard to Google. I would assume if a married couple were, say, given long-term sanctuary in a church building, or being sheltered there as refugees, they wouldn't be required to be celibate, even if the building only had the one room? Can't find any actual statements on it, though.

I am pretty sure any already-sinful sex is always made more sinful if you have it on consecrated ground, though. And committing adultery with an ordained priest who gave you a magical birth control amulet and is using Cathar doctrine as dirty talk during the act definitely counts as already-sinful in multiple ways. Though I don't get the impression Father Pierre was very creative with positions, and they did bring a mattress in! So maybe at least it was missionary position.