Not kidding, I had the same thing happen to me once. I was at work and this old couple with their grandkid came in and he was tugging on his grandma's sleeve telling her, "Look, that boy is dressed like a girl." He must've said it like five times. I didn't have the courage to say anything at the time, though, because I just wanted to disappear at that moment.
I'm so sorry you went through that. Instances like that, especially with children, are so, so painful, because they have no filters whatsoever and will just blurt the first thought that comes to mind.
You're not alone, sister :/ it's hard, especially when you feel like you can pass so well, and then someone says something stupid like that.