My GF and I have engaged in fun with other couples, but we haven't taken the plunge in play for pay. I have been in situations before where I saw my SO with women and men, and in combinations. I'm one of those people that delights in seeing others derive pleasure, whether I'm participating or not. It's something called
compersion.
https://en.wikipedia.org/wiki/Compersion
For myself, seeing someone I love have the time of their lives brings me joy. Jealousy, in my eye, is a vague term and a result of social engineering. Why are you jealous? Are you
insecure vis-a-vis the person that is with your love? Are you
envious of their time with them? Are you wishing
you had traits this extra partner has? Why? Your SO and you have such a shared history, so much time together, a built-up intimacy that only time can forge. This is just one more experience, and another way, another level, you can connect together.