That's only if the canon Six's build is the typical stocky guy or relatively thin girl in addition to their associated voices. Androgynous features and voice with neutral pronouns (they, them) could be a fabulous compromise for the sake of legitimizing both Dude and Fem sixes within canon. Unlike a character such as Commander Shepard (boob armor), Noble Six more or less wears the same armor regardless of the player's chosen gender. A good butt can be genderless. Another option is gender fluidity which would make even the simple switching of changing your Spartan's gender canon in a way. :P