Generally the people who bring it up are really used to being told to fuck off no matter how nice they are kindly asking people to please not use "gay" to mean bad. You sit on your hands and swallow your anger all the time and be really polite, and people still act like you pooed on their dog, and you get that enough times and you go, "FUCK YOU YOU ASSHOLE DON'T USE GAY LIKE THAT IT'S WAY HOMOPHOBIC GO DIE" regardless of whether it's a one-time screw up or not. That's really the basis of the tone argument, but it's also why people shouldn't be so sensitive to being told off.

I'm shocked every time someone responds nicely to when I politely ask them to stop doing something bigoted, really.