Men will instinctively try to protect women and put themselves in danger.

"The screams of women, they have a big psychological effect on men. A woman just has a different pitch," former Marine Carlos Laguna, who served to tours in Iraq, told Fox News in December. "If we're in a firefight and a woman is shot or lost her arm, male Marines like me would want to stop and help. It's our nature to help women."

It might be true that men have an instinct to help women. But a major part of military training is getting men and women to reverse the normal human instinct to, when things blow up all around you, get the hell out of there instead of do the fighting the military paid for. In fact, the military has spent quite a lot of money figuring out how to quell all kinds of natural instincts. In addition to the instinct of "not wanting to get shot at," there's also the instinct one might call "don't kill people." As Scientific American's John Horgan explained in 2010, "Surveys of WWII infantrymen carried out by U.S. Army Brig. Gen. S.L.A. Marshall found that only 15 to 20 percent had fired their weapons in combat, even when ordered to do so." Marshall argued that the average Joe had "an inner and usually unrealized resistance towards killing a fellow man" and would avoid it if at all possible. In On Killing, Dave Grossman, a former Army lieutenant colonel and West Point psychology professor, wrote that Marshall's findings were backed up by reports from World War I, the Civil War, and other wars. The military overcomes this with lots of training of body and mind. ("What makes the green grass grow? Blood! Blood makes the green grass grow.")

Women don't have the killer instinct.

Neither do most men. See above.

Women aren't strong enough.

Again, I am going to break some news here: Not every soldier in combat arms is huge. We imagine them all to be jacked like Wolverine, since we so often refer to them as superheroes. There are short guys, there are naturally less strong guys, there are bad runners. Some women might not be able to fireman-carry the biggest men, but neither can the smaller guys.

To pass the Army's PT test, you get a score based on the number of pushups you do in two minutes, the number of situps you do in two minutes, and how fast you run two miles. Slow runners compensate by running up the score in the strength tests, and the less-than-burly make up for it by running really fast. If you fail the weight test you can make up for it by passing the tape test, which measures the ratio of the circumference of your belly to that of your neck and shoulders. These tactics to get a passing grade will get you certified to serve, but it should be noted that none of these things match up very well with what you actually have to do in combat.

Women have periods!

People don't make this case much any more, because it is offensive. Newt Gingrich hinted at this in 1995, when he said "females have biological problems staying in a ditch for 30 days because they get infections." This case is made by Kingsley Browne in the book Co-ed Combat: The New Evidence That Women Shouldn't Fight the Nations Wars. Browne sites surveys in which women deployed in war zones felt unclean; he argues their anatomy creates an insurmountable burden. Here again I'm going to break some news: Men also pee. Men have anuses. Men have genitals. Men have balls, covered in delicate, sensitive skin. Men get rashes. Men get crotch rot. Men get "desert dick," although there is some conflict on the Internet about what that slang refers to.