Same way humans were differentiated by using races - ranking us lower/higher better/worse than others, that's what people do with species.
If you find the theory of evolution a fact, then you wouldn't laugh at me. Any rational evolutionary biologist will agree that species are here to simplify the conversations, they cannot be used for ranking which species are better - they are also not the main theme of research.
Do you know how species are defined? Sane definition? - members of the same species can produce a fertile offspring. But there are hundreds if not thousand of exceptions where this happens even across the species boundary - the reason why it happens is that our labeling is in its core subjective and somewhat arbitrary, there's no dividng line/plane/svm-hyperplane.
In the parallel lines of time in evolution and natural selection we've been as long as all the living beings.
You wouldn't find homo erectus (neanderthalensis, habilis, rudolfensis, ergaster, floresiensis, even some australopithecae) attractive but you could procreate. There's a biological reason why you can't have children with a horse, it's not a species reason, it's not even ingrained in our species labeling as can be seen from the above definition.
What, you would rank the being as irrelevant (or less relevant) if you can't have children with it? Comedy of irrationality.
Go try finding your rationality and get back to us on that.
I made that comment because of your assertion that species is a human construct, not because you are saying that species shouldn't be ranked as better/worse. Species is a biological classification, it is the largest classification at which two members can create fertile offspring, which is why I mentioned breeding in my comment. Trying to separate biology and species doesn't make any sense to me as the grouping is a description based on that biological compatibility.
I don't even have an argument with you because you have a consistent view on the issue, unlike those that eat pigs but not cats or dogs. So we're actually mostly in agreement, the arbitrary classification that people are making with pigs and dogs being food or pets is silly.
And no, breeding compatibility does not decide my diet.
Edit: Your third and final paragraphs were added after I started writing my comment, which is why I didn't address them. I also don't understand your fourth paragraph.
Yeah, I'm adding paragraphs because I'm trying to anticipate the arguments people put forth to rationalize appeal to nature - and draw their moral (rational) conclusions from it. Especially when they don't know that nature is defined with subjective and somewhat arbitrary definitions that have extreme amounts of exceptions. Same thing is done with religion, where there are plenty of definitions but hundreds of exceptions because definitions are not precise enough, and can't be.
As my third paragraph states, there are enough exceptions to see that species isn't always that definition, and that it is somewhat arbitrary.
Fourth paragraph is also an additional information for those who want to rank horses as lesser beings because we are obviously capable of reigning the planet thus our evolutionary path is more worth - without noticing that the evolution and natural selection don't have a goal set in stone.
If you don't have an argument, then why laugh at me with that horse joke? It's entirely irrelevant and, given my following comment, incorrect.
Race is a social construct because there is no scientific definition of race and is not consistent between cultures or even in the same culture over time.
Species however has a scientific definition - "A species is often defined as the largest group of organisms where two individuals are capable of reproducing fertile offspring." Thus not a social construct, this is a well defined classification. We may reclassify species based on new information or because they evolved, but that doesn't mean its a social construct.
Go try breeding with a horse and get back to us on that.