Without wanting to sound like any sort of ultimate authority (!), I don't think Barrie's letter to George wishing he were a girl had anything to do with fighting in the war, at least not in the context Barrie wrote it; in fact I think he more or less gives the reason himself: "I do seem to be sadder today than ever, and more and more wishing you were a girl of 21 instead of a boy, so that I could say the things to you that are now always in my heart."
In other words, had George been a girl, he could have told him/her that he loved him, whereas being a man it was "not the done thing" - NOT out of any fear of being thought a homosexual (I doubt that such things ever even occured to Barrie), but because British men of a certain class in those days (and still today!) had an absolute horror of revealing their emotions.
This theme was beautifully explored by Barrie in his one-act play, "The New Word", based on George's departure for the Western Front, which I quoted at length in my book...