While the military requires lots of strenuous physical activity that not every one would be able to hack, male and female, it isn't something so intense that only a small number of elite athletes could hang. It requires a good degree of physical fitness but it is no Ironman competition. Remember that these are standards that in major wars potentially millions of people have to meet. Any comparison to MMA fighters or Olympic athletes is way off base.
Physical fitness standards vary by service or country, but as a general rule they all are crafted to make sure that people are capable of doing both intense bursts and sustained periods of strenuous physical activity without getting winded (the running portion of physical fitness requirements), and some ability to lift yourself and whatever gear you might have to haul over obstacles and such (situps / pullups/ pushups, ect).
There are plenty of women capable of both meeting the physical requirements of those physical fitness tests and the tasks those PFTs are measuring strength and endurance for. Could every woman do it? No, but then neither could every man. I'm all for women having a shot at qualifying for combat arms specialties (infantry / armor / artillery). If they meet the standards and pass the same training courses as the men, why shouldn't they? The important thing however is to have the same standards.
Just wanted to add my .02 on the topic of women in the military, since some of the posts in this thread seemed to be of the 'they don't belong' variety.
Getting back to the topic of Mass Effect, I don't find it unrealistic that women are serving in the infantry in the Alliance. While I don't think there will ever be gender parity in combat arms specialties, I'd bet on most real world Western military organizations having women serving in combat arms specialties long before 2183.