Call me a geek, but in star trek they also refer to women (such as captain Janeway) as 'Sir'.
The reason for that, however, is to consciously and deliberately contrast the unisix future with current norms.
'Sir', in the English language, is solely used to refer to men, and always has been. I don't know about tamriel, but in Britain sir has always been masculine.
People say 'if there had been women knights back then, they would have been called sir'. That is an unintelligent argument because there women were deliberately barred from being knights. There is no point in asking the hypothetical question 'if there had been women knights' when beack then knighthood itself was gender specific.
In modern Britain, where the institution of knighthood still exists, female knights are not referred to as 'sir' but as 'dame'.
A further point is that 'sir' was a term used to address male authority figures. It was turned into an honourific given to a knight after it had become a common way of addressing male superiors (it derives from the middle French sire, which means 'my lord').
From the various game forums I have been to it also seems like a lot of people on game forums generally have trouble accomodating or supporting the female six, which is sad.