https://blogs.scientificamerican.com/observations/why-is-same-sex-sexual-behavior-so-common-in-animals/

For a very long time, scientists have known that animals engage in sexual behavior with individuals of the same sex. Such same-sex sexual behavior (SSB)* can include, for example, mounting, courting through songs and other signals, genital licking or releasing sperm, and has been observed in over 1,500 animal species, from primates to sea stars, bats to damselflies, snakes to nematode worms.

In recent decades, numerous hypotheses have been proposed and tested to understand why animals engage in these sexual behaviors that do not directly lead to reproduction. In a theoretical perspective published in Nature Ecology and Evolution, we reflect on the hypotheses proposed by biologists to explain SSB, and on the widespread but unquestioned assumptions that underlie them.