I have a friend with an open marriage,and he is always whining about his wife/lover/other guy.... All they are doing is inviting in more squabbles and hardship and heartache, so if they are doing it to 'save' their marriage, they've got another thing coming. If they can't make it work with one person, inviting in two more isn't going to help.
I don't believe for a second anyone is going to be truly happy in that situation... anyone who says their marriage or open marriages are always wonderful is full of it ;) They may have fun for a while, but isn't it really just a distraction of confronting more serious issues, while opening up a whole new set of issues, like rejection, jealousy, guilt, trust, shame, confusion...
What if one marriage partner switches his/her lover for someone else, and actually prefers that person over their spouse... or has a few more lovers than their spouse, and what if the spouse's lover doesn't work out and they are left alone? All sorts of troubles and rules to apply to a marriage. Not to mention, how confusing it would be for any children who are who will inevitably be involved.