> Because why else would such an aversion arise?

> I don't think there are any sensible "defaults" for human cultures.

But, you seem to think a lack of aversion to talking about sex to be a default? To your question, I've known many people whom are not practicing any religion and yet have an aversion to sexual discussion, within a population that has a lack thereof. There are many such topics that some feel are not keeping with decorum to be discussed openly and widely - and without religion being involved. Let's say in China there is a general aversion to sexual discussion. What will be your explanation given lack of puritanical religion?

> But I wouldn't expect aversion to talking any sex to arise spontaneously among a population

I don't see spontaneity to be relevant here.