From all the reading I have ever encountered, Homosexuality has been around for longer than the current version of Homo-sapiens.
I also understand it's not restricted just to Humans, but is more obvious with our species especially when "on display".
I would counter that it's not an abnormal condition, just an uncommon one you personally don't like, possibly due to religious belief's.
I think the only thing those statements demonstrate, is that
abnormal doesn't mean non-existent, nor new.
Depression has been around probably forever too, and I'm pretty sure my dog is depressed sometimes (LOL), that doesn't make it normal nor non-curable. But - and this is key - it DOES mean I have to put a
lot of effort into getting better, which usually feels like an impossibility to the sufferer and many are the moments they wish someone would just give them the OK to stay that way, as with any condition that requires work to cure.
Oh yeah, most of the time I'd love to wallow in my depression and point to the fact that many other people are depressed too and my brain supports my depression, as a way to avoid the rather hard work of changing.
I can point to the millions of years that any condition has been around and that gives no gas to the belief that it's normal.
With the way the reproductive systems of male and female are set up, it's obviously not normal to have the urge to stick your p___ in another man's butt, but nonetheless is a common-enough problem that some people do wish this.
You can study the history of how and why homosexuality was removed from the psychiatrists' Bible as a mental condition, it's quite fascinating, but boils down to something very simple - they were pressured hard, by a lot of people, who simply didn't wish it to be considered an abnormal mental condition any more.
You see something perhaps a little bit similar happening with Obesity today in the US: There is a push by some who simply don't like it referred to as a disease or an abnormality. Why? Same reasons you cite, basically. "It's really common, and it's really hard to change, and I'm fine with the way I am, in fact I like it, and so please stop calling it something that makes me feel a certain way about it."