> 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.
I genuinely don't know, that's why I asked. Presumably it's served some sort of purpose at some point. Or maybe, as another poster suggested, it was an trait borrowed from other cultures where puritanical religion did have an influence.