Polyamorous relationships would be my ideal, but I would say that they are very difficult to achieve in real life. I have always admired the way Robert Heinlein desrcibed his characters marraiges. This is best exemplified in Stranger in a Strange Land and The Past Through Tomorrow.

A poly relationship is more work because it can be so easy to get jealous, the more people involved, the more ways that little things can cause problems. I would love to have a poly marraige, but would hate to have the headaches.

A good resource can be found here: http://www.polyamory .org/SF/essays.html