Furthermore, mentioning homosexuality made me think. In 2003, Lawrence vs Texas, the Supreme Court ruled that "sexual activity between consenting adults is legal", and that attempts to make it not so were unconstitutional. That's been held up in the case of homosexuality, but (perhaps because in most states it is already not prosecuted much and a misdemeanor) it's never been tested.

So yes. It's illegal. In some states. Has largely been determined to be unconstitutional, your handwaving of "these people broke the law, they get no sympathy" notwithstanding.

Yes, because no politician wants to be the one who says "I propose a bill to make adultery legal. It's not an issue for the courts."

There have been less that a dozen misdemeanor prosecutions in the last half century in those 21 states combined.

So yes, in those states, it's illegal. Just as in many cases, so was homosexuality, and anal sex.