The first time this issue ever actually became apparent to me was in a conversation with my dad about violent video games. I was playing Resident Evil (the GameCube remake) at the time, and I remember him making an off-hand comment, which was definitely meant as a joke, that he was less worried about violent games making me violent and more worried about the fact that I was playing as a girl. I'd honestly never thought about it until that point.
Over the years I've played a whole lot of games, and I've definitely played more female characters than male characters (excluding games where you have no choice for the player character's gender). I've always had some kind of rationale for it, though.
Like in the first Resident Evil I'll play as Jill Valentine, because I've always gotten the vibe that her story is the one that the series considers canon. In Mass Effect I'll play as FemShep, because Jennifer Hale's voice acting is a hell of a lot better and more believable than the guy that phoned in the MaleShep lines. Stuff like that.
When it comes to MMORPGs though, it was mostly about aesthetics. I played WoW for years, and nearly all of my characters have been female because I just couldn't stand the look of the male character models or their clunky animations. Sorry, but when I'm playing a caster... seeing a 300 pound pile of muscle and anger dressed in a robe making sparkly jazz hands just looks wrong to me. Seeing the same 300lb pile of muscle and anger dressed in leather with tiny little knives trying to be stealthy looks just as wrong. If the game had more variety in body types, I probably would have played more male characters.
The Old Republic actually offers different body types for each gender, and go figure, now I have an even mix of male and female characters. I'll use one gender for the first advanced class for each main class, and the other gender for the second advanced class. /shrug.
I don't really think players using the opposite gender for their characters really says much about them. The times where it does are likely the exception rather than the rule.