If we eliminate the idea of people as property, what are we left with? Every individual as a free agent, who can decide who they want to be with, live with, be intimate with. Those who want exclusivity are welcome to choose that. Those who need other arrangements are welcome to choose that. All that's required is complete honesty.
The idea of jealousy has to go, of course, and that's a significant hurdle. But that's an idea rooted in possessiveness and ownership. I don't own you, so you're welcome and encouraged to to what makes you happy. We all are.
If we say that the root emotion behind relationships is love, then shouldn't I want you to have the experiences you feel you need? If I don't want that, it sounds like we're talking about control, possession, and fear - not love.
Once we start thinking this way, some bad words disappear from the language.
"Unfaithful": there's no such thing. Whatever you're doing is, by definition, being faithful to yourself.If you treat everyone in your life with honesty and integrity, I don't see how this wouldn't work, and how there wouldn't be a quantum leap in happiness for everyone, and for society as a whole.
"Cheating": also no such thing. You decide on the experiences you want, as you don't belong to anyone else.
"Adultery": another concept based in human ownership.
"Vows": this concept has to stop or change form, as we have to realize that we can't promise how we're going to feel tomorrow. You can't make a promise today and have both people assume it's good forever after - the process of reevaluation has to be ongoing.
"Guilt": there's no reason to feel guilty, you're doing what you feel you need to do, which is perfectly natural and acceptable.
"Shame": as with guilt, there's no need to feel this way. Enjoy your sexuality, revel in it.
Of course I'm playing devil's advocate here, and am well aware that this would be a huge leap in consciousness for a lot of people - but I really do believe that it would be a better way.