In Bruce Bagemihl's seminal work on this subject, 'Biological Exuberance', he cites that in black swans, homosexual pairings between male swans, which were lifelong, had higher rates of chick survival than heterosexual swan couples.
The males would mate with a female, nest, and then when all of the eggs were laid, drive the female away and incubate the eggs themselves.
The resulting cygnets enjoyed greater survival rates because male swans are more aggressive than females, and as a result, paired male swans were able to command larger and better territories in which to raise their offspring, who benefited from having two bigger, stronger defenders.

Conversely, female paired black headed gulls suffered lower rates of chick survival because BOTH females would lay, and the resulting super sized clutch made successful incubation difficult, as only one female at a time would incubate while the other was out feeding, and she could not cover all of the eggs with her body for proper incubation.

If you have not read this book, I strongly recommend it, as it gives all sorts of fascinating information on not only homosexuality in animals, but other aspects of sexual activity, reproduction, and non-reproduction among a wide range of animals.

Homosexuality is natural, sex is natural, and can be done for both pleasure and for reproduction.

We're just the only animals who insist on making the whole thing more complicated than it actually has to be... as if we don't have enough reasons to go out and be terrible to one another.