Go to a cattle yard. Cows mount each other all the time. Whether that's sexual or some other behaviour (eg dominance-submission) I don't know.
Anyway to back up AngeliqueEast's point - I too have read of reports of homosexuality in animals. There was some work done some time ago into the behaviour of dolphins. Group sex, threesomes, rape, homosexuality - it was a veritable cetacean orgy (Spongebob Squarepants probably got in there somewhere
) - all observed by scientists.
There have been reports of homosexual penguins which mate for life and which even mimic heterosexual penguins in "parental" relationships. Scientists have observed one male penguin treating a rock like an egg while the other partner penguin acted in a complementary manner (eg getting the food and so on).