I don’t know who badgerigar is, but his comment was flagged and killed, even though it is scientifially correct.
Although animals have been observed to have same-sex coupling, they mix it up with males and females. In the animal kingdom the only species that have been observed to have exclusively homosexual sex are humans and domesticated sheep. One would think that a sexual organism never having heterosexual sex would be a huge evolutionary fitness disadvantage, and such a trait would be extremely uncommon. There are some caveats, for example: Sharks are able to reproduce asexually, and frogs can “switch” genders AFTER reproduction, during adverse conditions. But lifelong homosexuality has not been observed in species other than humans and domesticated sheep, which suggests that relaxed evolutionary pressures may be a necessary component.
From Wikipedia
Simon LeVay introduced caveat that "[a]lthough homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity."[8] One species in which exclusive homosexual orientation occurs, however, is that of domesticated sheep (Ovis aries).[9][10] "About 10% of rams (males), refuse to mate with ewes (females) but do readily mate with other rams."[10]