My native language is spanish so I might be having the wrong impression on english but I have heard "sir" being used in a formal manner on ocassion with both genders.

I might also be wrong here but usually when I dont know someone´s gender, I just say them politely about they would prefer to be addressed as. I dont need to dwell in details and its clear (or just omit terms). Afterall, verbal language is not the only way to show respect to others.