"They started out with a fairly largely component of the men thinking this is not a good idea, and women will not be able to do this," he said in an interview with NPR.
"When you start out with that mindset, you're almost presupposing the outcome," he said.
The report, released Thursday, is part of the Marine Corps's efforts to study integrating women into the infantry in order to meet a 2012 order by then-Defense Secretary Leon Panetta to open all military jobs to women.
Services must open all jobs to women by January, or else submit requests for exceptions by the end of this month. Defense Secretary Ash Carter will then review the requests and make final decisions by early next year.
The study showed that females in the unit were injured twice as often as men, were less accurate in shooting and were not as good at removing wounded troops off the battlefield.
Mabus argued that other studies, including one by the Center for Naval Analysis, say there are ways to mitigate gaps in performance "so you have the same combat effectiveness, the same lethality, which is crucial."
Mabus said some of the report's conclusions were based on generalizations and not the women's performance.
"Part of the study said women tend not to be able to carry as heavy a load for as long, but there were women who went through the study who could," he said.
"And part of the study said we're afraid because women get injured more frequently that over time, women will break down more, that you'll begin to lose your combat effectiveness over time.
"That was not shown in the study, that was an extrapolation based on injury rates," he said.
However, he praised the setting of standards by the service for combat jobs.
For example, now there are set standards for Marine artillerymen, "in terms of the rounds you have to lift, in terms of the numbers you have to do, in terms of the firing rates you have to make, in terms of armor, the same thing with rounds in a tank."
"None of those existed before, you would just say, here you go, you're an artilleryman," he said. "Everybody will have to be at the same level, they will have to meet these standards that have now been set that simply didn't exist before. Once you do that, there shouldn't be these gaps."