Twenty or so years ago my wife and I dabbled a bit in the swinging lifestyle. Not extensively, as it was difficult to find other couples with mutual attraction to us, and timing and scheduling were always an issue.
We were successful three times with other couples, and we both enjoyed it greatly. There were a few jealous pangs for each of us, but for the most part our love and mutual trust helped us deal with it.
The first time watching my wife was akin to an out of body experience. I was greatly tuned in to her reactions as he took his time undressing her, kissing and touching her, and eventually laying naked with her - as I stood naked next to the bed watching. We maintained a lot of eye contact as they coupled, and our unspoken communication was erotically intense. When the vocal responses began I understood from a different standpoint what she was experiencing.
Most interesting for me was my feelings as her partner spread her legs and mounted her, and I had a beautiful view of his slick cock slowly enter her. I knew what he was feeling, but a lifelong curiosity had me lusting to experience that penetration for myself.
Later, after returning to our own room, we discussed everything at length, I assured her I was okay with what had happened, and we had some of the best sex of our lives.