I know there are other lists of slashy science fiction and fantasy, but I'm going to make one anyway, because I've read a lot, and some of these authors are out of print and I want them to get fans. (How's that for a motive?)
For example, Jane Fancher is, to me, the slashiest of the slashy, particularly her sf "Groundties" series. Also her fantasy series "Dance of the Rings," which is slashy but in a less-sex and more male-bonding way, because the main characters are three brothers. (natlyn, it was three, wasn't it?)
C. S. Friedman's Coldfire trilogy: BLACK SUN RISING, WHEN TRUE NIGHT FALLS, and CROWN OF SHADOWS. Cue evil immortal antihero Gerald Tarrant and his stalwart, morally superior, forced-into-working-with-him Damien.
Ann Tonsor Zeddies, Deathgift and its sequel, Sky Road. The first book seems like fantasy until the characters come up hard against superior technology. These are out of print, and I confess I love Ann, but that isn't why I love these books, I found Deathgift and loved it before I ever met her. They are so, so slashy. Go and find them, and read them.
Susan R. Matthews' "Judiciary" series is the slashiest one can get long before there is any gay sex, as there is in the most recent book. We're talking male bonding up the wazoo here. Angst and pain and lots of culturally sanctioned non-sexual male/male kissing. If you can get through the parts about horrible torture, these are so slashy.
I have a few more, but I need to confirm titles and such.