An animal model of spontaneous exclusive homosexuality has however been described in sheep. About 8% of the males in a population studied in the western United States were shown to mate exclusively with other males, even when the choice was given between a male or female partner (Perkins and Roselli, 2007; Roselli et al., 2011b).
Never is a strong word when that’s just not true
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7484171/
Huh, wasn’t aware of that. Well, after a bit more research, excluding that one species in that one study, there are no exclusively homosexual animals.
You see how a lack of scientific reporting on the subject makes it less likely that you find more examples, right?