"Because, after all, insisting on monogamy even when it makes your spouse unhappy or even at the cost of dishonesty (the REAL killer of marriages) isn't really love at all. It's possession."
- This probably explains the situation a lot more easily than I've been trying to. Relationships which don't involve trust or some diligent attempt at transparent honesty will fail. It's possible that relationships would fail on the basis of one person honestly wanting to sleep with someone else, if for example it makes one or both people uncomfortable to the point of legitimate unhappiness. It is more sensible simply to not get married to each other where one party cannot accept this possibility or this as even an option (ie, the possession view), that is to say, they would not "love unconditionally" and accept the nature of the other person involved in their affairs.
But I'd suspect they're a lot more likely to fail because one or both people dishonestly do lots of other things of which their dishonest sexual infidelity is merely yet another symptom. So to my mind, those seem like some sensible rules (listed up in the post above), even if they're never needed or called into question over the course of a relationship. There are plenty of people who don't consider those going in and must grapple with the probability that their significant other ends up bedding up with somebody else and then having to discover this in a rather shocking or unpleasant fashion. Sometimes that relationship recovers from that shock, people discuss, forgive, and hopefully move on with only bruises to their egos or some time rebuilding trust. Sometimes it doesn't, usually where people become paranoid and dominating instead of attempting understanding. I'd rather know going in that I'm dealing with a human being and that implies I don't have absolute control and absolute power over them. Influence is great, sometimes it even forms a basis of control. By influence, we might mean that each party is concerned over the welfare of the other and takes care to place it equal if not in principle to that of their own. This surrenders some abstract level of self-identity, but in practice probably frees the individual to do more with what they have to hand. This is quite distinct from power. Power like that which demands rather than seeks compliance, like that of an absolute tyrant, isn't great.
Real power doesn't need to be exercised and demonstrated to be effective.