On our 10 year anniversary we stayed at a B&B. Part of the anniversary package included a massage for the wife.
At this time, I had never told my wife about my wife sharing fantasies, so it was quite thrilling for me when a masseur arrived at our room. As he set up his table, he gave my wife instructions to undress to her level of comfort, and come out with a towel wrapped around her.
I was with my wife in the bathroom, and encouraged her to get completely naked -- I reasoned she wouldn't have to worry about getting lotion on her underwear, and he wouldn't have to work around them. I left the premises while she got the massage. So I'm driving around the town knowing my wife is completely naked in a room, while another man is rubbing and touching her.
Knowing my wife, I knew nothing was going to happen, but the idea that it could if she wanted it to, and there was nothing I could do to stop it since I wasn't there, was almost intoxicating. At the appointed time, I came back to the room so horny I had to fuck her immediately.
That was a long time ago and my reluctant wife still hasn't been shared. I would love to do something like that again, but with a masseur who is experienced in the art of erotic massage, and my wife now aware that it can go as far as she wants it to.