Some creationists seem to assume that X and Y Chromosomes apply to all animals and even plants. (When they don't even define sex in all mammals.)
I think I understand why. I used to assume X/Y set sex in animals (or at least mammals or perhaps quadrupeds) and I was *never* a creationist and learned all of the things that lead me to that incorrect inference while still a believer. (Though belief in god was not part of that misguided inference.) In fact I'm still not sure how far the X/Y system extends in the animal kingdom.
These are the things I think lead to the inference in my case (not in order of importance or chronology):
1) I learned about simple Mendelian genetics
2) I learned about the human sex chromosome system
3) I understood sex determination in animals to be largely genetic (with known exceptions for determination by temperature, etc. in some non-mammals) in other vertebrates
4) and where I had access to that information (domestic mammals) from experience, I saw that the ratios of male to female newborns was roughly 1:1 like in humans.
Thus I inferred that there was not only a single Mendelian sex chromosome switch in mammals (and probably other quadrupeds) but that it was the same X/Y system in humans.
The thing is I *still* don't know how far it extends. I think I only heard about other sex determination genetics in the last decade, and certainly after my last biology course (which was last century). Perhaps even after my last major dive into the pop-sci evolution/anti-creationist books I have quite a few of. (Don't recall any of them discussing the evolution of sex determination or frankly much about the evolution of sexual reproduction.) I may have even learned about them from posts on this board.
Though I don't actually know how far the X/Y system extends I will take a guess that it is not unique to humans or that would have come up in the discussion of human evolution (like the chromosome 2 fusion event), but I don't know if it extends to the rest of the primates.
[Since this was a post about my non-creationist sourced ignorance on this topic, I didn't try to fill the gaps in my knowledge with an internet search, thus maintaining the effect such that I could write the post without distortion. Now I'm going to have to find out...]