This is a valid argument
I think it depends whether they can develop attraction to one sex, which is probably debatable. For example, I definitely wouldn't call my girls straight- they definitely do what they do for "love"- not dominance. They take turns standing on eachother and its entirely consensual on both parts.
But if they had a drake, they may decide the drake is fulfilling their needs better and not bother with eachother anymore.
So I wouldn't say most animals are straight or gay, they're just normal, natural and acting on instincts and hormones. If that makes sense? Or perhaps it's a spectrum for animals similarly for people, and most are somewhere inbetween? I don't know.
But I do believe that some of the more intelligent species have the capacity to have strong preference. So I still hold the view that they "can" be gay, in theory.
Maybe scientists will learn definitive answers, either way, someday
Basically where I stand on the matter. Gender and sexuality are weird and fluid, even for humans, animals act upon their instincts a lot more, so technically most animals would have engaged in "the gay stuff" at least once