There's a book from 2000 called Biological Exuberance: Animal Homosexuality and Natural Diversity that has data on this very subject. And yeah, many animals are functionally bisexual or homosexual for part of their lives and then mate with the opposite sex, then go back to being homosexual, there are recorded instances of individuals of a species refusing to have sex with a member of the opposite sex, choosing only to mate with the same sex. So they're gay. Some excerpts from the book:

Bonobos: "In addition, females have sometimes been observed consistently ignoring males soliciting them for sex, preferring instead to GG-rub with each other."

Chimpanzees: "However, a few individuals appear to be more exclusively homosexual: one female, for example, refused to mate with males and was only involved with other females for many years" (This one did eventually mate with males but still there were some like this that never did)

Macaques: "In other troops, though, some females are exclusively lesbian, engaging in sexual interactions only with females: in these cases, an average of 9 percent of females are homosexual, 56 percent bisexual, and 35 percent exclusively heterosexual."

Red Fox: "Many female Red Foxes that mount other females may be exclusively same-sex oriented, since such younger or lower-ranking individuals usually do not mate with males. For some females, this homosexual orientation may be longlasting—perhaps even continuing for a female’s entire life—since as many as 50–70 percent of vixens never leave their home groups to begin breeding on their own."