The fact that the default skin is a guy (Steve) doesn't mean the game itself is sexist. Look at all the other games. Some have male chars, some have female ones.
As for the behaviour of people on minecraft servers, it's just how majority of teenage boys act. It doesn't always mean they really are sexist but you know, on the internet some people become horrible, unlike the way they are in real.
World created by the media is a bit of sexist nowadays, so i'd blame that rather than this awesome game
I used to run a successful server in which one of the MAIN rules were: no to sexism, racism, or discrimination based on any reason. And it was a really important rule, you could get easily banned for that. Few examples given made it so that when a new player started breaking those rules, older players were scolding at him themselves when i wasn't around, telling him the consequences
