I started mentioning to my wife that I wanted her to get fucked by another guy. She never dismissed it, but was surprised. Once during sex I brought it up, and she was receptive to the idea. As we got into it more and more, I asked if she had a person in mind. She said she did. I told her to close her eyes, and imagine it was his cock in her instead of mine. After a little bit I asked who it was, she she seemed reluctant to tell me. I eventually convinced her, and she said "Mark." I asked, "what do you want to do to Mark?" Again she was reluctant, so I was just like, "say it." Then I heard the 5 words I still jerk off to..."I want to fuck Mark."
Fast forward a couple of days and I'm playing with her pussy in bed. She doesn't seem super into it, and her pussy is dry. Then she asked me, "what do you want me to do?" I was confused, but she asked the question again, somewhat suggestively. "What do you want me to do?" I said, "I want you to fuck Mark." She moaned and there was instant rush of moisture to her pussy.
She has told me it's something she'd consider as long as I'm ok with it. I am, of course. The interesting part is she hangs out with him, sometimes at her place. They text all the time. I'm not stupid, I know what's happening and I love it.