Attempt at explanation:

the prevalent culture is very much not to regard sex as sacred in that sense

That's how that culture "wants" people to think (believe-in-belief) - because sex-in-itself has largely the "taboo" + "big" + "powerful" factors for it so people shrink from thinking too much about it; while the "importance" and the "emotional charge" factors are attached to the intersection of sex and romantic love/marriage/childbirth. Thinking about sex with those attachments is easier in such a culture than thinking about sex-in-itself.

Presumably to make sex without those latter attachments less desirable/less of a goal on a memetic level, but keep the overall cover of sacredness.

Sounds plausible/falsifiable enough within MixedNuts' model, doesn't it?