halacha - Why has Judaism traditionally discussed intimacy only privately?

To quote msh210's formulation of this site's rule on intimate topics:

Please respect that in the Jewish tradition certain questions, especially certain questions relating to sexuality, are discussed only in private. Such questions will be closed or deleted at the discretion of the moderators or community.

This formulation has since become site policy, and it has been added to the help pages and the sexuality's tag wiki.

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First, what sources record this idea?

Furthermore, why is it our tradition that these topics be kept private? What reasons have our sages given for doing so? Is this tradition halachic or minhagic in nature?

(Sources please!)

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^{(The original meta question on this site policy has recently seen some new activity, which prompted this question, to be used as a resource for any further discussion that might occur.)}