If I could reorder the entire mindset of the human race and undo millenia of social conditioning, I would change the "rules" so that a person's life partner and their sexual partner are in no way expected to be the same person.

Don't get me wrong, I have no problem with marriage and monogamy - this is the direction in which I choose to lead my own life. But when you think about it, a spouse fulfills two very different functions, and when looked at detachedly they don't necessarily seem complementary.

One is the life partner - a person with whom you share your household and finances and families and the trials and tribulations of day to day life. The life partner is practically a business contract.

The other function is the lover, which is self-explanatory.

Now I know in many many cases couples manage to successfully amalgamate these two functions, but if you think about it from a detached, purely logical perspective, it's a bit extreme to expect a person who can fulfill your needs as a life partner to also be able to fulfill your needs as a lover, and vice versa. There is a certain degree of love required to trust someone enough to be your life partner, of course, but a platonic life-partner relationship could carry on quite well without the kind of love generally associated with a sexual relationship. For example, I can think of about five people I know personally with whom I could plausibly be life partners for as much of eternity as I can conceptualize at age 22. But I can only think of one person whom I would choose as a permanent sexual partner. A life partner requires so much more that has nothing to do with romance - agreeing on managing money, compatible standards of cleanliness, buying furniture, dealing with insurance, even sleeping (by which I mean sleeping) in the same bed - that it's practically unreasonable that the pool of people who can meet these requirements has to be narrowed down to people who would also make compatible lovers. Promiscuity wouldn't need to be a necessary characteristic of this society, a pair of lovers could be monogamous if they wanted and monogamy could even be mainstream. Your perma-lover simply wouldn't have to also be your life partners.

The other problem here would be what to do about children, since they originate from sex but are part of a family and household. Since we're reordering the universe anyway, I think the simplest solution would be to make society matrilineal and matriarchical. And because the concept of sexual fidelity to one's life partner wouldn't exist, the fathers wouldn't be as bothered by the fact that they're sharing the household with another man's child. Of course, there would be more people involved in the decision of whether to have a child, but we'll deal with that when we come to it.

And no, I'm not condoning adultery (unless, of course, it's by mutual consent of all parties involved, and even then I still find it distasteful). I'm talking about a hypothetical reordered society. In our current society spouses, by definition, are expected to fulfill both these functions for their partners, and anyone who doesn't want to do so shouldn't enter into that sort of relationship. In the reordered society the concept of adultery wouldn't exist for the same reason that in our society there is no word for consensual sex within a maritial relationship that has the "sinful" connotation.