unless "help, I've fallen in love with the general of the of the invading army" suddenly counts as an LGBT issue

I would say that all "issues," done right, are universal and human issues. Love between enemies pits personal feelings against group loyalty regardless of the genders of the characters. Even a tale about LGBT tolerance, if well told, wouldn't be a simplistic Very Special Episode saying "X are people, too!" with X being gay and trans* in this case. Rather, the story would be about fear, morality, identity, pain, rage, visibility, power, and belonging, told through the specific characters and setting of LGBT people in a certain society.