The headline is unfair to his argument, I think: it suggests that he is opposed to women serving. That's not actually what he said at all.
Murder is illegal on the streets of America, but people still do it. So sexual crimes are still going to happen in the military regardless of how they [Pentagon and Congress] try to clamp down on it or revise [policies]. It’s still going to happen and at that point, I don’t really question the physicality of women. I question the logistics of the Marine Corps and if they will be able to keep up with it...This would be a strong point if and only if the complications of integration are going to be long term problems and not (as with racial integration) short term problems; and if and only if those complications are going to make the Marine Corps less effective as a fighting force. Are either of those 'if and only if' criteria implausible?
“Sen. Elizabeth Warren was a prime example, pushing for a co-ed boot camp and she questioned the commandant on why we’re not doing that,” he told the Daily Beast. “So I think it’s coming down from civilian leadership and I think the Marine Corps should stand up for itself and say, ‘Look, we have been winning wars for America since 1775. Why should we continue to change the wheels when they’re not broke? We need to look at American defense first before looking at integration.”