I'm not saying we should limit ourselves to the best of the best and not recruit anyone else. But I am saying that every soldier is aught to strive to become the best of the best. Of course not everyone is equal, some soldiers will be better than others. The best of the best will get promotions and will be put into front-line and other key positions in the army, while the lesser capable soldiers will be put in support roles.
Sure, women are good enough to join the army, but are they good enough for front-line and other key positions? In real life, the answer is no. In Mass Effect, the answer is: I don't know, but probably not. Why do I think that? Because outside of femShep, I haven't seen a single female soldier pull of any extraordinary feat in Mass Effect. Shepard is obviously going to be an exception, because from a gameplay standpoint, it would be incredibly lame to make a female Shepard weaker than a male Shepard.
I don't disagree with you that every soldier in the alliance military, man or woman, should push themselves to be as good as they can, physically. I'm saying that between the enhancements they receive and the difference between peak human and above average anything else, the difference between human men and women becomes negligible, so there's no reason to keep them off the front lines.
As for female human combatants, it's true we don't see much in the way of the extraordinary from anyone but Shepard, but here's a few examples of women who either demonstrably or implicitly excell in the ME universe.
Firstly, and obviously, there's Ashley. Granted we don't see anything exceptionally remarkable from her in cut scenes, but she's good enough for Shepard's squad and, in ME1, is the single best offensive team mate.
In Kasumi's personal mission, we see her vastly exceeding what the human body should be capable of when she takes down Hock's ship; it's never exactly explained how, but she does. Suppose that could just have been the animators having a laugh, though.
For implicit, out of the four confirmed N7 graduates in the series not including Shepard, two of them are women. Their roles are very minor compared to the known male graduates - Kai Leng and Anderson - but just the fact that they made N7 says something.