If most people don't implicitly think sex is ridiculous, foolish, and shameful, why don't they have sex openly, in public, like animals? Why are exhibitionists the exceptions that prove the rule?

I think you'll say the openness would spoil the intimacy, assuming the sex is with a beloved partner. But part of that emotional intimacy is the secret the couple carries about their sex lives. What adds to their intimacy is the trust they share, since they realize they alone could blackmail each other, knowing what they do about their sex acts.

Thus, when you appeal to intimacy, you're conceding my point since if sex weren't ridiculous, there would be no way to blackmail anyone with evidence of their sex life.