General Robert Neller promised to change the Marine culture while testifying before a Senate committee.
Last week, reports emerged that current and former Marines were sharing photos on Facebook and on message boards, triggering a Navy investigation.
But female members of the Senate panel pushed back on Mr Neller's promises.
Senator Kirsten Gillibrand, a New York Democrat, said that the military had not done enough to address longstanding allegations of rampant sexual assault and harassment.
