It tells us that he's a human being first, a teacher, a wizard, an expert on dark creatures, a person who makes mistakes -- all these things first, and then he also happens to be gay. It tells us that being gay isn't the end-all-be-all of someone's personality and life experience. It tells us that there isn't a great divide in the world with all the gay people conveniently Over There on their proper shelf where you don't have to see them (separate but equal).

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. You know? He goes to McGonagall's office for detention and waits while she smooches Sinistra goodbye or something. You know, he knows, it's not a surprise, he doesn't care, readers aren't supposed to be shocked or alarmed, and the people who are already burning the books because of witchcraft, well, it's not incredibly ikely they're allowing their children to be corrupted by this in the first place, now is it.

This is, incidentally, one of the things I love about Mercedes Lackey's Valdemar books, which people say are bad, and maybe they are, but in her first trilogy the main character isn't gay, but a pair of her friends are and she knows it, supports it, and doesn't care, in the sense of, it's not relevant to the relationship she and her friends have. And in a later trilogy, hell, the main character is gay, and again, it's not all he is. Those books, incidentally, are usually classified as juvie lit.