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.
1. Physically isolating females from males.
2. Conditioning females so they won't seek these opportunities.
In combination, these factors seem to taboo any discussion of sex at all in mixed male/female company. It seems our standards for what is "family friendly" grows out of these taboos. You'll notice that in exclusively male company discussing sex is generally much less taboo.
With the obviously problematic morality aside, this does seem like the most effective approach to assuring paternity, particularly in small, low-tech, tribal groups.
Edit: There's also the need to limit sexual violence, which also seems to be a factor in tabooing discussion of sex in mixed company.