How do we know that homophobia doesn't exist in other species?

I keep seeing this quote “Homosexuality is found in over 1500 species and homophobia is only found in one. Which one do you think is unnatural?” and various permutations thereof.

What I want to know is how do we know that homophobia isn’t found in other species? Have scientific studies been done? Have animals been observed around other animals that are proven to be homosexual?

Note: Please do not turn this into a homosexuality vs. homophobia debate. I just want to know if we actually know if homophobia doesn’t exist in other species.