An understanding of commitment is often still there, yet an understanding of wanting to try other people is also there, if it is mutual then that's a choice.
Marriages don't fail from orgies or partner swapping, or foursomes or watching someone with your partner if that is their choice and there is a clear understanding.
You are talking about "cheating" where one person is un-aware of the other, sleeping around or with someone else.. That is totally different.. There marriages do fail..
Together?they never want to root themselves in some intimate relationships and they want to go beyond.
Love can still be had with partners that want to share experiences.
There are different people in this world.
Like I said, you are talking 'CHEATING'