Ursula K. Le Guin, Left Hand of Darkness

http://en.wikipedia.org/wiki/Gay_science_fiction

Samuel R. Delany is by far one of the better known homosexual scifi / fantasy writers. He is also African American, and has commented that when being asked to speak on a panel, the moderators often can’t decide to group him with the blacks, gays, or scifi writers.

His characters often have homoerotic encounters or thoughts in his work, though never to a pornographic extent

Nicola Griffith, Gael Baudino, John Varley (Titan, Wizard, Demon)

Russell T Davies – Doctor Who

I’m unsure if you meant “gay science fiction writers” or just writers who write “gay science fiction.” But here is my contribution:
David Gerrald (not sure of his sexual preference), best known for writing the Star Trek episode The Trouble with Tribbles, has written a few stories that include homoerotic themes. One of which, The Man Who Folded Himself has a man going back a day in time, and forward a day in time, to have sex with “himselves.” Interesting gay mènage à trois.

How bout Dorian Grey ..