Thomas Bayard, a former senator and secretary of state, became the first American diplomat to be given the title “ambassador” when he was sent to London in 1893. Since then a few thousand people have had that honor. But only three of them have been openly gay. That is about to change. Last month, President Obama put forward the names of five openly gay nominees for ambassadorial posts. It’s a remarkable step, though possibly less revolutionary than it appears. For while the nominations reflect the change in public attitudes toward the LGBT community, they also show the importance of domestic...