I've known of some people (no one here, by the way - just to clarify) who deliberately adopt a male persona if they are female, or a female persona if they are male, simply because it gives them a kick to mislead as to gender online...
I've known some who simply don't care. They're themselves, and that's enough for them online.
The internet offers an unprecedented opportunity to mask oneself. For instance, how do you know that I don't actually have short green hair, a hooked nose, dentures and withered claws? ... Oh well, I suppose that, if I did, the people I've met in person here would mention it. *grin*