I agree with susanne and would say that most of the animal species dont copulate for enjoyment but as susanne said are homonal driven (a species of monkey are an exception but i cant remember which) for the rest it is mostly in order to reproduce and also as said a dominance / social structure reasons.
I wouldnt say you can class animals as gay thinking of it in the same way as humans, we can be attracted physically and emotionally to the same sex and want to pair bond with one possibly, animals dont tend to be attracted through looks or emotion but want to mate when and as the hormones dictate.