Yep, happens to me occasionally as well, and strangely, more often in summer when it's pretty obvious I'm NOT a guy. Yeesh. I've never really been girly-girl thanks in part to a military upbringing, and being tall doesn't help either, but even modestly endowed as I am I occasionally get the 'sir' thing.
Before I retired from the optical biz, when I got called 'sir' I'd just offer the offender my business card (imprinted with my first name Sherry, and my title)and suggest (in a serious but tongue-in-cheek sort of way) they come see me for an eye exam, or at least new glasses!
I've never really liked my first name...think it's a little too sucky for me...but using the business card trick works every time.
I still keep a card with me for those who are suffering apparent visual difficulties.
All vintage, all the time.
Falcon Black Diamond
Gitane Tour de France
Kuwahara Sierra Grande MTB
Bianchi Super Grizzly MTB