[Pseudoscience-is-malarkey (OP)]

A Russian colleague of mine told me today homosexual behavior is unnatural as you'll never find it in non-humans. I know he's wrong, but has it ever been seriously studied?

[Kryptid]

It's been studied in bonobos.

[Colin2B]

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

[alancalverd]

A friend has published studies on "third sex" behavior in some African birds and I'm told that dolphins consider humans somewhat repressed. 

[Bored chemist]

has it ever been seriously studied?

Yes.
Notably.
Another irresistible quotation comes from the Los Angeles Reader and
might be headlined Publish Or Be Damned. ‘University of California, Davis,
graduate student Anne Perkins, on her study of the sexuality of sheep: ‘It
is very difficult to look at the possibility of lesbian sheep because if
you are a female sheep, what you do to solicit sex is to stand still. Maybe
there is a female sheep out there really wanting another female, but there’s
just no way for us to know it.’
https://www.newscientist.com/article/mg12817427-000-ariadne/

[Pseudoscience-is-malarkey (OP)]

It's been studied in bonobos.

Hmmm. You know, I encountered him at a casino one time while he was raising funds for his Africa charity, and I felt a chill. He must have been undressing me with his eyes.