X-Men, at least the main teams, should be mutants only. That's because the whole point of the X-Men it's to be a mutant team, first to show mutants as heroes, then to protect mutantkind, and a lot more mutant-related stuff. But Avengers or whatever, they're not about the members being human. They have all kinds of species, humans, aliens, gods... So there's no reason mutants would be excluded.