I have two stories for this. Both completely different.

My Fiancee and I often have a “Open relationship” with sex when we’re both feeling like fucking with another individual. Its something we both wanted to do when we first started dating and it actually never occurred until we both decided to have a threesome. We had a blast, Now she has one once a year and I might join her if i’m feeling it. We love eachother, I don’t mind. We talked about it before the relationship and it worked. Once we’re married we will both stop though, having fun now. Never after.

My previous Ex wanted one, I said no. She cheated, I kicked her out of my home and threw her stuff on the curb. Lets just say she went into a downwards spiral and I had to get a restraining order on her. Pretty sure she OD a few years ago.