Many people criticize the concept of an "open relationship", that is, a relationship in which both partners are allowed to have sexual relations with people other than the primary partner. There are also other forms of so-called "polyamory", for example a three-way relationship which excludes sexual relations with anybody besides the other two partners. While in some cases such relationships may only benefit one party, may involve coercion or neglect, or sacrificing for one's partner, there are some such relationships in which both or all partners find themselves more fulfilled and happy than they otherwise would.
Yet these "good" polyamorous relationships are the subject of the same moral aversion and disdain as the abusive, coercive ones. What kind of moral argument could lie behind the idea that such relationships are wrong - surely not a morality based on happiness. Is some kind of deontological sexual ethic at the root of the criticism of open relationships and polyamory? What does this ethic look like? Why, in short, is there opposition to polyamorous relationships that goes beyond mere personal disagreement?