I would research the law, just to be 100% certain it's on your side (it can vary by state). Then I'd keep a copy of it and shove that under their noses every time they said something, but I'm a smartass like that. 🙂
Other than that, I'm glad to see you're mostly over it, but I'm with everyone else : a woman uses the woman's bathroom, period. Imagine how intrusive and offensive it would be - to cis people too! - if they examined everybody's genitalia at the door. Failing that, in the *women's* room at least nobody sees what is in anyone else's pants (it may be that, as guys, they just don't realize how much privacy there is in there?!).