Because this is an issue of military fitness and the good order and discipline of military units, there should be a strong presumption that those decisions be made by the people in uniform. The “chickenhawk” argument that nobody can have an opinion on military matters if they haven’t served is dumb and offensive on a lot of levels. Decisions of war, peace, and national security are too important to the nation to be left only to the soldiers and generals. But there’s an important lesson here on experts in general. Like any group of experts, the military needs to be left to manage its day-to-day affairs without a lot of outside interference, but also needs civilian oversight and subordination to civilian authority on the major, big-picture decisions. The question of fitness to serve is much more in the day-to-day affairs area, so the rest of us should generally stay out of it.
That distinction has long been one that a lot of Republicans respected. It’s why there was a lot of GOP opposition to Bill Clinton when he tried to introduce openly serving gays in the military over the objections of a lot of the military; by the time the “Don’t Ask, Don’t Tell” policy was repealed in 2010, the military was less unified on the issue, and while most Republicans (including McCain) still opposed legislation repealing it, the arguments still tended to focus on whether the support for repeal by President Obama’s Joint Chiefs of Staff was representative of military opinion on the matter. Here, by contrast, DoD is looking into the issue, and most Republican senators are — properly — uncomfortable having to stake out a position without much backing or cover from the uniformed services. Thus, even military veterans like McCain and Joni Ernst are unwilling to give Trump their support on this without some backup from the military.