As far as gender issues, that is not a SECDEF issue. He can make recommendations, but it is Congress who needs to act, and that isn't going to happen when a sick, biblically illiterate citizenry keep voting idiots into office. So it is the whole nation's fault. Not one man.
The President is the Commander in Chief, not the congress and the Sec Def is his point man for the military. Trump could issue an executive order tomorrow to separate women from combat arms and likely would do so if wet sponge Mattis would recommend it. Trump should have fired every three and four star officer on his first day in office and brought back all the Generals and Admirals purged by Obama. But he hasnt drained the swamp in DC nor the Pentagon and the swamp is doing all it can to destroy him.