The Significant Other Penguin and I fall under Rule #1 because we do not believe that monogamy is the correct choice. I have seen far too many people do what you and your wife failed to do - maintain that choice made in public in front of friends and family. Worse, I have seen people deviate from that choice, destroy their relationship and still stay together. Monogamy is not a natural state.

Instead, our agreement and understanding works like this - we will stay together as long as we're happier together than we would be apart. If we are unhappy then we talk about why, thus far we have decided to stay together. With regards everyone else in our life, each relationship we have with other people is ours to determine, those relationships can be sexual or not, but they are ours to manage. I believe it's more than possible to love multiple people at the same time - nobody seems to suggest that 'love' (whatever it is conceptually) is diminished when children come into the picture, so I don't see why it would be by another adult.

Micro-cheating happens all the time, by both of us, in the open. Macro-cheating also happens, sometimes in the open sometimes not - but it isn't cheating. Cheating implies we've broken rules we've never set ourselves. We are coming up for 13 years together, multiple other monogamous and committed friends have since divorced after trying to get us on board with the good ship monogamy.