Animals can't be gay, lesbian, trans, bi, or anything.
Perhaps they could be, but how would you know? They'd have to be able to explain how and why they have a preference for one sex or another or both, and animals aren't able to do that. If indeed they really even understand gender at all.
As Shadrach pointed out, some animals will even form a relationship of sorts with an inanimate object, so I wouldn't put too much stock into who or what they may or may not find attractive.