You say that 'the most' we could say about it is a chemical imbalance? I doubt you were actually trying to be bigoted, but that's how that comment comes across... who's to say it's an *im*balance and never provided a selective benefit to one's genes? Who's to say that a brain that makes you find men more desirable than women is a 'bad' thing? That is essentially what your friend is attempting to do: cite a vague idea of natural selection and an appeal to nature.
First, the appeal to nature is fallacious and easy to point out to your coworker, as they are almost certainly inconsistent on using it. They likely sit in a nice, comfortable office and do novel things that don't further their chances of copulation *all the time*. Take away their video games because they're 'unnatural' and he might realize the hypocrisy in his reasoning.
Actually, that's all you need. I won't list a second reason
. Your friend is literally thinking that sex for having babies = good and natural, sex not for having babies = bad and unnatural and is of course only applying this in the context of homosexuality and not any of his own activities or other views. We can only hope that he doesn't have other latent fears or rationales behind his homophobia and will come around once he learns of this.
Sorry for posting so late!