Birth rates wouldn't historically have been a big differentiator. There are edge-cases, I suppose (some early Christian sects encouraged complete celibacy, for everyone!) but in general effective contraception wasn't available, and even where it _did_ exist (Romans _may_ have had some sort of somewhat effective contraceptive/abortifacient drug, say), it didn't attract all that much religious interest until the modern era.

This makes a lot of sense from a perspective where the "gods" are essentially ancient AI running on distributed (neural) hardware and communicating with itself via our standard human sensory channels that are able to hijack our wetware in proportion to our devotion. Monotheism then becomes a form of centralization of processing, restricting societies to a (more) unitary direction of intent [at the cost of much waste at the level of individual processing units (people)].

A slight tangent to your slight tangent: Monastic regimens look an awful lot like proof-of-work... Is the next step in facilitating the human-machine meta-minds minting AI crypto-religions?

Also, taking into account the earlier thread on Sci-Fi vs Sci-Fantasy, with a comment mentioning that science fiction tends to be current social commentary in the guise of the future: self-aware (and self-minting) currency a la Accelerando is an interesting analogy for religious, spiritual, cultural, and economic movements.