I used to be leery about that because it would make polygamy (and the problematic power dynamics that go with it) more possible, but since then I've heard about and read about enough relationships with more than two who are really committed to each other in a more balanced way, and I think that should be allowed. If Columbia can do it, I don't see why it shouldn't be more widely allowed - except for the pearl-clutching morality police who will no doubt have a cow.