Originally Posted by Dark Raven
Namor is a mutant too, but he doesn't belong in the X-Men.
I don't think it should come down to a particular characteristic you were born with that determines where you should go anyway. It's not that far off from people of different nationalities. If there is an "international" student group of Asians somewhere, should people automatically stick someone who looks Asian in that group as well if he or she has been raised and brought up in a Western culture and only speaks English? What people usually mean by these "international groups" is one full of foreign students for whom English is not their first language. The Asian brought up in the West might appear visually the same and have certain common traits, but he or she has been brought up and nurtured completely differently and would have trouble fitting in and communicating.
I voted that the twins should be with the Avengers, where they've spent the majority of their existence associated with that team.
Namor worked for 30 years before getting that mutant origin, you don't need to explain much, he's the son of a human and an atlantean, having the mutant gene is just a bonus for the character