What makes couples decide to be in an open relationship after they were monogamous for a long time?

I know some monogamous couples that are now in Open relationships and open to swinging as well, They all said they are happy with each other and it made their relationship better than what it used to be to be before. It shocks me when I meet couples that are this way , The one couple wanted to swing with me and my girlfriend at the time and we both said No way , I said I wouldn’t be in a relationship with anyone , if they wanted to go screw someone else period. To me that is just FWB’s , So Would you stay with your spouse , if they all of a sudden wanted an open relationship with you, or would you end your marriage immediately?
What makes couples decide to be in an open relationship after they were monogamous for a long time?
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