Is aversion to discussion of sex a part of traditional Chinese culture? Seems odd given I'm not aware of any puritanical religions taking hold there.
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There are loads of sensible "defaults" for human cultures. Aversion and disgust at the practices of unfamiliar out-groups is one - keeps us from getting their diseases. Practices assuring paternity are another - males that are indifferent to who's children they raise aren't very well selected for. Risk aversion in, and preference for protection of, child-bearing females by the group is a third - harm to these females disproportionately affects the ability of the group to reproduce and pass its genes. There are many, many others, and we have many of them in common with our animal relatives.