Whatever else evolutionary biology says, the only thing that matters in my view is that homosexuality is not a trait limited to homo sapiens but is present in a number of other species, too, both within and without the mammalian class.
Apart from that, the chimpanzee vs bonobo discussion is ever so faintly ridiculous in my opinion.
We split from our common ancestor 6.5 million years ago. All of human society evolved after that. Recent genome analysis has also shown that we are equally closely related to either species, neither is closer than the other actually.
I do believe that the argument regarding a closer relationship of humans to bonobos has in my view far more to do with wishful thinking than anything else.
Chimpanzee society is firmly, violently patriarchal. They are often brutal, hunting down weaker members of the species with adult males fully in control and females and juveniles treated accordingly.
Bonobo society in contrast is seen as far less aggressive, matriarchal, bisexual and polyamorous. Conflict is typically solved not by violence but by seeking to release tensions via close physical contact like face-to-face sex, tongue kissing and mutual grooming, often ending in orgies involving most if not all members of the group.
That desire to get away from the thought of humans, specifically the male of the species, being naturally violent and having evolved naturally to be killers is attractive to me too.
(Most research on bonobos should come with a disclaimer that almost all of the knowledge we have on them is based on observing them in captivity. Researchers positing ideas about the "peaceful" bonobos usually point to the fact that the same is true regarding our knowledge of chimpanzees. The flaw in that argument is what this really means is that with much of our knowledge coming from captive populations of either species, all we can really say for sure is that we know how they behave in captivity.)