Animals just do not think like humans, so we cannot consider them gay, straight or bi.
Certainly they seem to have favourite sexes, horses and even colours, with which they like to be with. Mounting in animals, is not necessarily a sexual thing. More often than not, it is just a dominance issue. This especially true in dogs who 'hump'.
Some males who have been severely scolded by mares in the past, will buddy up to geldings for life.
Some seem to like certain colouring. One Gypsy Horse who came from England to quarantine, refused to breed mares there. Stallions in quarantine from overseas, must prove they can breed and settle a mare. This stallion's bill was mounting for the new owner, because he refused to breed the mares there. Jeff Bartko, had the thought to bring a Gypsy mare to the quarantine station and the stallion was immediately willing to breed her. Who knows if it was just the colour, or if he recognised his own kind. Those whom he had been brought up with.
My daughter had a Gypsy stallion, who was crazy about one particular mare. She hated him. Poor boy, even resorted to offering her his best toy ball. At one point, the silly, love-sick boy, offered her a folding chair! He would take these gifts to her stall door and stand there, hoping I suppose, for even a little encouragement. He was never asked to breed her and was very quiet and patient about the whole thing. Strangely enough, a colt we had, also for some reason fell in love with this particular mare from the time he was born. She never 'shows' to any horses and really isn't the least bit interested in making friends with any. However, it does seem that the boys of all ages, pick her out as being special. Who knows what is so attractive about her.