Homosexuality as a sexual dysfunction

I have quite a heated debate about homosexuality. The guy I'm debating with technically is not a homophobe. He accepts homosexuality is not a psychiatric disease, he supports equal rights for gays but he still insists homosexuality is a sexuality dysfunction because gays can not naturally procreate (so procreation function is kinda broken). The same way colour-blind people are totally okay socially-wise but their eyesight is pathological.

But this is not an old boring "they can not procreate so it's wrong" argument. He doesn't think bisexuality is a sexuality pathology as they can procreate (so he's pretty consistent in this regards). You can not argue that with a typical answer about child-free people as he is talking about a biological pathology, not social choice. Old people and infertile people (disease) also kinda support his point.

The best response I came to is that procreation is not a mandatory biological function for a human being. There're many beings in nature that are born not to procreate but to help for procreation (gay uncle hypothesis and increased amount of babies among gay person relatives fullfill this analogy).

Are there any better answers? Or is it fair to say homosexuality is a sexuality dysfunction? Have there been any scientists (psychiatrists?) who treated bisexuality as something normal and homosexuality as something dysfunctional (at a bilogical level)?

I'm not sure why you'd bother to debate it. When someone has such an limited understanding of sociology and biology. That'd they'd come up with such a bigoted stance. They're going to cling to it irrationally. No amount of argument is likely to budge them. And it's possible to support gay rights and still be a homophobe. As this guy is demonstrating.

It IS an old boring argument. His TL;DR is that the only way to define a "functional" sexuality is to produce children. That argument has no basis in nature. Then, that the only way to produce children is to have penis in vagina sex. Also ridiculous. Any divergence from that "natural path." Is therefore classed as a dysfunction.

Honestly. I think you can find better things to do with your time.