Yes, but various forms of non-heterosexual sexual behaviors do exist and serve various social functions throughout the animal kingdom. Primates and canines particularly tend to use it as a hierarchical social indicator. Certain species of snakes employ bisexual behaviors as a means of increasing their chances of mating with a female.
Homosexuality may be observed in different species, but it is not at all the 'default' orientation of any species. Homosexuality is rare within any population. It is stupid to assume that heterosexuality is not the default orientation of any species.
Bonobos may be a bad example for *cough* other reasons, but they're the first example that springs to mind - individuals with a sole interest in one gender or the other are rare among bonobo populations; by behavior, they're largely bisexual.
Why you always lying~
Hmm hmm
Oh my gosh, stop fucking lying~