How come heatran has sexes (not ‘genders’) and can’t breed? It’s the only legendary that is like this (Latias and Latios have fixed sexes, Manaphy can breed).
Well, Latias and Latios probably have genders because they’re supposed to reference Gnostic aeons, which come in male/female pairs (I’m not altogether convinced by this interpretation, but it’s the best suggestion I’ve seen for what they’re about); Manaphy can breed because being a legendary Pokémon that can breed is Manaphy’s particular gimmick. Heatran… well, to be honest my best guess (and I do not think it is a terribly good one, but it’s what I’ve got) is that Heatran was originally not intended to be a legendary Pokémon, and somehow its genders were retained by mistake. There isn’t really anything particularly “legendary” about Heatran other than its stats, after all, particularly not in comparison to some of the other nonsense going on in the fourth generation. If you’re looking for an in-universe explanation, I would say that Heatran can and do breed – but since they’ll only lay their eggs in a volcano that isn’t already claimed by another Heatran, good bloody luck to anyone who wants to try it.