Navy Secretary Ray Mabus defended his decision on Tuesday to open all combat positions in the Navy and Marine Corps to women, despite the Marine Corps asking to keep some front-line positions closed.

Mabus said that an oft-cited Marine Corps study that found women performed worse than men in combat situations looked at the average woman, not at individual capabilities.

"There were — and are — capable women who can meet the arduous standards the Marine Corps set for ground combat arms units. We all know the Marines have never been about average," Mabus said at a Senate Armed Services Committee hearing.

But Sen. John McCain, R-Ariz., criticized those who chose to brush the results of that study under the rug, hammering Mabus without naming him.

"The Army and Marine Corps studies demonstrated that biological differences between men and women can have implications," McCain said in his opening statement. "Rather than honestly confront these realities, some have sought to minimize them."

Both service leaders and lawmakers stressed that standards can not be changed to accommodate women.

"No quotas and no pressure," Army Chief of Staff Gen. Mark Milley said.