I think monogamy is a kind of... artificial (?) condition.  I've been trying to maintain an open relationship for about two years based on that principle:  it's natural for us to be attracted to other people and it's not wrong to act on it, but in a relationship our sexual decisions necessarily affect both partners, so that has to be taken into consideration.

Sometimes it works, but working out the details takes a lot of discussion, sometimes argument.

So we're trying to train ourselves in the ideals of openness, but we're no more equipped for that than we are to be exclusive, and I often ask myself whether it wouldn't be easier to default to monogamy just to minimize the conflict.  ; )

I'd love to hear from others who have more success maintaining variations on conventional relationships!