Not wishing to labour the point, but I found this on Wikipaedia:
In most modern Anglophone countries, the groom extends this honor to someone who is close to him, . . . When the groom wishes to give this honor to a woman, she may be termed the best woman or best person, although traditionally she would still be referred to as the 'best man'.
I just think it's fun to be called the best man . . .
My husband also had a woman in that role when we got married, which was much more unusual all those years ago!
Jayne