Right on Marla, I'll second that emotion!
It's the mixed presentation which is at work here. Even in full female presentation, many Muggles will still slip up and call us "sir" because their brains detect and perceived in this order...
female
no wait, male
oh, it's one of those TG types
So now knowing how to address TG because we're all new in their world, they revert to the last one in the hierarchy, male. Voila, here comes the "sir".
But there is a point which was not brought up. What if everyone called you "ma'am??? Wouldn't that rub you the wrong way as well?