Nope, people can do what they want. But they should be free to do so without hurting another. If a relationship is exclusive, or at least started out that way, then the person who wants to stray should tie up loose ends first, and give the person they are with a choice - will you share me, or not? At least then there are options for the person who may not actually want their partner to sleep with others and allow them to find someone else who wants exclusivity like themselves. And I disagree, nobody 'keeps partners against their will, unless in an abusive relationship. People are free to go.

Lying, cheating, sneaking are not a way to treat someone who you are with. So you could say there is no justification to hurt someone either.

Its very interesting how you objectify sex and relationships. Its almost like a business deal with you I think? Forgive me if am wrong. Problem is there are many many people who do not feel the same. I am one of those people, and I find that an exclusive relationship isn't just about safe sex, its also about trust, love, hopes, dreams, sharing a common goal with relationships, being tight knit. Without those things, sex is just sex, it is just an act. And sex like that for me is empty. As they say, why have a burger when you can have steak? He's my best friend and I don't think I could feel the same about him if he were sleeping with someone else, or I was.