It normally comes up when claims are made of the form "homosexuality is unnatural!" with the implied or explicit "therefore it is wrong/sinful/evil/yucky". Pointing to same-sex pairings in animals is intended as a response to this. The people making the response either don't understand the naturalistic fallacy or consider it to be sufficiently abstract or harder to explain that they don't bother with that line of response.

It is also interesting from a biological standpoint in that it isn't that easy to explain from an ev bio perspective, so studying it makes sense.

It has to do with the fact that it was essentially ignored throughout most of the history of biology as a discipline. It's not like this behavior is new; it's been there the whole time, and so have the observations of the behavior, but the reaction within scientific culture has changed dramatically.

Stuff like interpreting active vs passive animals in a copulatory act as male and female respectively, assuming the animals had simply misidentified the sex of the other party, or assuming that the observing party was necessarily mistaken, publication and citation biases, and the frequently-opaque titles, abstracts and contents of those published studies that did manage to make it into the journals ("A Note on the Apparent Lowering of Moral Standards in the Lepidoptera", W.J. Tenant, 1987, Entemologists Record and Journal of Variation).

It's news to a whole lot of people, in other words.

Wild mass guessing: animals are incapable of sin?