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.