SPOILERS FOR MOST RECENT EPISODE AND HARSH CRITICISM. YOU HAVE BEEN WARNED
Diana's comments on the topic: http://www.outlandertvnews.com/2015/04/diana-gabaldon-clarifies-black-jack-randalls-sexual-orientation/
HOWEVER IN THE OUTLANDISH COMPANION HE IS CALLED "A SADISTIC BISEXUAL PERVERT" So......hmmmm