The problem from a roleplaying perspective is that a female character, say a Sister of Battle or assassin, cannot participate in the cohesion system which is integral to the game mechanics of Deathwatch.  What is supposed to be at the core of Deathwatch this both a mechanical and roleplaying story of coming together, unifying and overcoming the foes of humanity.  A Sisters of Battle focused game on the other hand could have male characters exhibit traits such as faith because Dark Heresy allows for men to gain faith through alternative classes or through purchase of the talent.  So yes a man cannot be a Sister of Battle, but can be real damn close.  A female cannot be a Space Marine and cannot make anything comparatively close or even able to mechanically play alongside Deathwatch character.