I am absolutely happy for people to live the life they want to as long as they do so without harming others, but on a professional communication the writer's gender preferences are totally irrelevant to the matter in hand and have no place on these letters.
It might as well say: John Smith, likes jazz - it is nothing to do with the professional approach.
My DGC knows that their preferences are their own and they never get on a bandwagon forcing these into people's faces. They just quietly get on with their life. They have my support to be themself.