I think it's fallacious to argue that the only way two opposite-sex characters can be confirmed to be in a Platonic relationship is if they're currently seeing other people. That's not the way it works. A Platonic relationship isn't just the lack of romantic opportunity. There are a lot of different ways that people can bond, even if they're of opposite sexes. It's simplistic to assume the only possibilities are "in a romance" and "not free to be in a romance." After all, it's not like every man is attracted to every woman, or vice-versa. Sometimes -- probably often -- there's just no romantic chemistry at all between two people, but they can trust and respect each other as friends or rely on each other as professional partners.