Actually I can weigh in on this with personal experience.
The issues that come out of this are mostly from our views on sex and love as a society.
My wife and I have a pretty open relationship. We both have permission to pursue any actions with anyone we want, as long as there is complete open and honest disclosure. This doesn't mean we don't love each other. I would never leave my wife for anyone, she would never leave me. And trust me, people have tried to get us to.
Jealousy is a big issue. Not between us, but other people. We haven't actually done much with anyone else because honestly, most people aren't well adjusted enough. I've had several women go all crazy stalked on me without even knowing I have a pretty open relationship. Makes me kinda gun shy.
Our view of sex has been molded to say "You shouldn't have premarrital sex, and you should never stray outside your marriage."
Ok, that's fine if you're of a Faith that says no and you believe in that go ahead.
In my opinion people put WAY too much emphasis on sexual relations and the word love. To the point that I don't think people know what love is anymore.
I personally believe that one must decide for themselves. I do not see it being as much of an issue, considering many couples cheat on each other anyway. Our country loves to make money off of this. Talk shows that focus on cheating spouses and lie detector tests.
And websites that have slogans saying "have an affair" are quite common.
Personally I see a commited and loving relationship with an agreement to be honest and open to possibilities to be much more civilized.
Feel free to ask any questions you want about the subject. Keep in mind we are not singers, we don't have constant random sex.