Putting women in combat arms in the US military has already unquestionably lowered its combat effectiveness. Arguing otherwise is lunacy. What's more, it has also lowered the nation's effective casualty tolerance, which is yet another 'force divisor'. In addition, it will predictably result in the lowering of standards---you don't SERIOUSLY think women flunking out Marine Infantry Officer training will be tolerated very long will you?---and not just for women. We're not capable of a least-hard honest quota system, we're way too insane for that.
But, honestly, I'm not terribly upset about this. Consider, who is the US military likely to be used against in the future? Can you think of any conflicts it might get used in that are beneficial to the interests of non-elite white people that aren't dead certain to go NBC on us? Also, hardly out of the question is that institution being used Janissary-style against us. So weakening and desacralizing the institution now is probably actually to the good. The existing order does NOT deserve the courage and sacrifice of men like the two former SEALs who gave the last full measure of their devotion to employees of a State department that holds their like in contempt. This is one of the reasons reactionaries---and frankly libertarians and conservatives as well---should support truly profound cuts in the military budget--at least 50% and probably more than that. Not only can't we afford it, it is extremely dangerous and creates undue temptations to go off in quest of monsters to slay.