You might try "Brothers of the Watch" by Miri Thompson on LiveJournal. The main characters are best friends (one straight, one gay) that work and live together in an original fantasy world about masters, slaves and a murder mystery. Part of a much larger verse called Halcrest. Not what I would call a hardcore slavefic, but one that focuses on gentle, romantic sex scenes (fade to grey) between the gay lovers (the gay best friend (Shane) is in love with his friend's younger brother (Devon)). Warning for references to a rape scene though. Miri's a great writer!
JP