Some ethical truths are intuitively obvious but it is hard to understand the reasons for them. For instance, sexual behavior should be, at least other things being equal, kept private. But why? While I certainly have this intuition, I have always found it deeply puzzling, especially since privacy is opposed to the value of knowledge and hence always requires a special justification.
But here is a line of thought that makes sense to me now. There is a natural connection between the sacred and the ritually hidden recognized across many religions. Think, for instance, of how the holiest prayers of the Tridentine Mass are said inaudibly by the priest, or the veiling of the Holy of Holies in the Temple of Jerusalem, or the mystery religions. The sacred is a kind of mysterium tremendum et fascinans, and ritual hiddenness expresses the mysteriousness of the sacred particularly aptly.
If sexuality is sacred—say, because of its connection with the generation of life, and given the sacredness of human life—then it is unsurprising if it is particularly appropriately engaged in in a context that involves ritual hiddenness.
Note that this is actually more of a ritual hiddenness than an actual secrecy. The fact of sex is not a secret in the case of a married couple, just as the content of the inaudible prayers of the Tridentine Mass is printed publicly in missals, but it is ritually hidden.
I wonder, too, if reflection on ritual hiddenness might not potentially help with the “problem of hiddenness”.