I get that this is a stupid marketing ploy to have young adolescent boys stare at this page for a fraction of a second longer than some other random page filled with text, but I wouldn't call the add sexist
I tested the theory that it is simply a bad advert by showing it to my wife.
Her reaction was not "OMG, that's soooo Sexist" but instead the more expected "LOL, that's gross ! What does her being on the toilet have to do with gaming?"
I think maybe some people jump on the sexist bandwagon because you can't approach the ASA with a complaint that the advert is stupid and expect to succeed.