I think if you haven't both taken the precautions to permanently no longer have children (unless that's part of the plan, have a baby, and one partner can't so you're asking someone outside the marriage to help and they agree) then yes, it's crazy, irresponsible, stupid, a good way to fuck everyone in the situation up (especially the babies).
Outside that, as long as they're honest, open, and follow their own rules then whatever. None of my business either way as long as they aren't hurting other people. Better be using condoms so they aren't spreading shit, but that's just basic common sense and human decency.
Do you! I think people have the right and responsibility to do what they want, desire, and anything they deem healthy and happiness-inducing. If an open marriage is that then cool. I can, to a degree, see how you connect with someone deeply and love them, but maybe are both busy and spend time apart and therefore satisfy sexual needs with others at time. Or maybe they love multiple people, but one mainly. You can have separate, unique, rich, rewarding relationships of different types and levels with multiple people. I have a friend that is poly. She has a main partner she lives with and who is only with her, but then has multiple other relationships from basically just friends to cuddle buddies to friends with benefits to playmates/fuck buddies. It works for all of them, so more power to them in my opinion.