This is not really hypocrisy, it's actually the way belief often works. An example of this is an Israeli politican who is an atheist. She was asked if there was any problem with her being an atheist, and she said "I don't believe in God, but I believe in the Jewish people, and the Jewish people believe in God." Or maybe that you don't think there's anything wrong with infidelity, but you are outraged that someone made it public because it will corrupt the morals of the youth or something. It's even possible for no-one to literally believe that infidelity is wrong, and yet the belief still works because everyone assumes that someone else believes and wants to preserve their belief. This explains the phenomenon of a pastor of a church who is an atheist, but nonetheless every Sunday promises the congregation they will be together in the afterlife. In order for belief to be operative in a culture, it's not necessary that people actually believe, only that they assume someone else does.

It is, as you say, the way belief often works; this doesn't make it not hypocrisy, it just means humans are often hypocrites. (Though, I get the idea that the use of that term on LW and OB may differ a bit from the usual usage; here there does not seem to be any implication of conscious or deliberate hypocrisy. This may in fact just be a definitional dispute.)

I'm not claiming that the actual rule is "Don't get caught." If that were the case, the authorities would simply say that. This kind of rule is like the rule against public nudity: you are allowed to be nude, just do it discreetly, in private. You are only prohibited from letting yourself be seen nude by others, from being caught. Or other examples, like urinating, defecating, etc. No-one thinks that people aren't ever naked, or that they don't defecate, we know it's just done privately. People use euphemisms and say they are going to the "restroom" (to rest?), but we know what they are doing, they just don't say it directly.

This doesn't seem to be analogous. People are explicit about the fact that the rules are "don't go naked in public", etc., and not "don't go naked". This is not the case with non-monogamy.