I think that Konrad Lorenz, the ‘father of ethology’, stated that other than man, there are no homosexual animals.
However, there are animals which exhibit bisexual behavior when something has gone wrong in their social order or development.
Some animals other than man DO, sometimes, try to mate with others of their own sex.
I can think of abnormal situations, where a lot of one sex are all that is available, where homosexual behavior happens. But as a rule, if a stallion has a choice between another stallion and a mare, he chooses the mare.
Of course, most mammals mate to create offspring. It isn’t a prudent lion who tries to take a lioness out of season. I had a mare bred to a stallion. He was an experienced sort and waited until the mare went, in a few days, from “Hell no!” to “Hi sailor! Looking for a good time?”