I was going to bring up the use of Ms. myself, but precisely to point out that it gained ground, not through laws, but the normal way, i.e. through discussion and convincing.

As for the argument that "it's just courtesy", that's also exactly the point.

The State has no business legally mandating courtesy, politeness, manners, etc., that's overreach that creates resentment. 

People complain about other people being rude for all kinds of reasons, people disagree on what constitutes rudeness, they write to advice columnists about it all the time, negotiating social rules is what all societies do, but free societies don't pass laws mandating politeness, "or else"...