There's also the infamous Toy Biz, Inc. v. United States case where Marvel Comics argued that their X-Men toys where "nonhuman toys" and not dolls to avoid an import tax.
Still, technically true though I'd think. Mutants aren't human. The X-Gene makes them "Homo sapiens superior" and distinguishes them from humans who were mutated by other means (radiation, spider bites, etc). Do X-men want to erase the distinction or do they just want peaceful coexistence between the two species?
I'm not saying this would affect a court ruling, but as a point of curiosity: technically "human" means the homo genus, not just the sapiens species. So Neanderthals for example, or homo erectus, are both human. The X-men would be human too, if they were indeed homo sapiens superior.
Mutants and humans can produce fertile offspring, but hybrids aren't impossible across species just uncommon. Bison and cattle, or grizzlies and polar bears are examples. Where we divide groups into "species" is pretty murky. I'm sure somewhere there has to be a biologist who loves comics enough to sort all this stuff out for us. Then again, I've seen Neanderthals called both a subspecies and a separate species of human too so they should probably sort that one out first.
As species boundaries go, human/mutant is incredibly arbitrary. Either a mutant or a human can be created from a pairing of human-human, mutant-human, or mutant-mutant. It would be like classifying blonde people as a different species.
