There's a clear line in this case: A species is a group of any living beings that can produce fertile offspring.
Donkeys and Horses are separate species because they breed and produce Mules which cannot reproduce.
My main argument was that the OP's supposedly clear-cut genetic category was not defined. You also seem to be saying that it's not clear-cut and defined. So why are you saying that my argument is silly, as opposed to the OP's?
> “ethnicity”
Neither the OP nor I mentioned this word, of course. You're shifting the goalposts. One could pick out a certain set of genetic markers, and one could do a clear-cut classification based on those. I'm firmly of the opinion that one should not, but in any case, that would be about genetics (as the OP and I discussed), and not about this word that you are trying to drag into this to deliberately muddy the waters.
> You can make the exact same argument about “species”
Proof by analogy is fraud, as they say. Even if the analogy holds. This one doesn't: There are fairly good ways of defining species objectively. For example, as the second sentence of https://en.wikipedia.org/wiki/Species says: "It can be defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction." I'm sure that there are a few exceptions around the edges, but this is much more clear-cut than any human divisions that you're trying to drag into this.
Or consider looking at it from the other direction - it only even seems to make sense if you have some preconceived notion about species anyway. If two humans are infertile or have an infertile child, does that make them different species from each other? Of course not.
I’m not making a novel argument, Darwin goes in-depth on this very topic in the Origin of Species.
“Species” and “ethnicity” (to which “ancient Roman” belongs) are in fact identical concepts; the only difference is the zoom setting on populations, which in both cases varies for any specific example but generally is farther out for species and closer in for ethnicity. In both cases they are arbitrary lines drawn around real clustering of population genetics.
There is an is-ought distinction. We can reason, investigate, and speculate about the ancient Roman people and their distinctiveness and what populations today have the closest genetic distance without there being any implication whatsoever socially or legally.