I imagine if she makes someone explicitly gay, it won't be Remus.

For all the reasons you give here, I do wish she would put in a passing reference to someone being gay--someone whose sex life is patently unimportant to Harry, who knows.

Yeah, she probably won't do it, but if she did, I wouldn't be surprised if it were Tonks. We know very little about her, and she's on the periphery of Harry's life - not important enough to be center-stage, but still a character people like. She's also not already a symbol of anything.