A young boy enters a church, and finds the nun(s) to be nicer than he had expected.
A somewhat short prompt, you may need to wrangle it a bit at the beginning if on small models.
This was initially intended to have a focus on impregnation, but putting anything about it in Memory or Author's note turns the story sexual very quickly, especially early on. I'd recommend letting it develop for a while before adding anything like "and the idea of a holy woman becoming pregnant" to the end of the AN, or editing the last line of Memory to add something similar.