For almost the first half of our marriage I believed in the myth of eternal monogamy and that having sex with only the same person for the rest of one's life was the established norm and goal. Any other concept seemed alien to me due to possessiveness and strong jealousy should I even think about other guy showing interest in my wife, let alone seeing her nude.
It wasn't till I could acknowledge those feelings and truly feel enough trust that she was not going to leave me just because someone else found her attractive that I could even broach the issue of bringing others into our sex life. In other words, separate the pure physical of sex from the pure emotional and commitment of love and marriage.
Even then, our first encounters were limited to the wife just playing with other women or having sex with just each other in front of other couples, as neither of us still had the comfort level to see each other with opposite-gender strangers. Eventually, like many things, that fell to the wayside as both the trust and separation of sex and love had been more of a gradual process than a sudden revelation. Of course in recent years all those issues have been sorted out and tidied up to where both of us can acknowledge and be comfortable with our sexual expression with others.
None of this has proved detrimental to our relationship (and in fact has strengthened it) as what we have with each other we will always have and is far, far more important than the single sexual experience with another. Having gotten to see her with other guys, but knowing it is me that she will still be going home with afterwards and having been through so much together, exclusive to us, has also helped my acceptance.
Just the fact all human beings think and look at others with at least the occasional sexual thought, proves to me, monogamy is not the natural state, but merely something society created for what they considered the good of the family and religion.
This is not to say, one shouldn't strive for it's practice most of the time as commitment is the cornerstone to any relationship and not everyone (maybe most) could come to post-enlightenment swinging and make it work; but if it is a mutual consent thing, neither should it be looked down upon.
So maybe due to my personal experience and journey my view is biased, but I see nothing wrong with either choice as long as one partner is not forcing it on the other and putting their sexual needs or desires above each other's.