I've been married three times and shared them all. Nothing like the first time though.
A friend of mine picked us up from the airport on our return home from vacation in Jamaica. She and I had often talked about her fucking someone else, and she had said she "would" but it didn't seem likely to ever happen. We invited him in for drinks when we got home and I noticed that she started acting flirty towards him, which was turning me on.
She modeled a wrap that she'd bought in Jamaica, which was pretty revealing, but for modesty, she had left her bathing suit on underneath. I told her it didn't look right that way and she went to the bedroom to see if she could fix it. She was gone a long time, so I finally went to check on her. When I opened the door, she looked at me like I had caught her doing something. She was wearing the wrap without the bathing suit, and indeed it did reveal quite a bit. You could see her nipples clearly through the fabric as well as her full 1980's bush. She said, "See? It shows too much." I assured her that it was fine but she was unconvinced.
I went over to give her a hug and then snuck a finger under the wrap, between her legs. Her pussy was absolutely drenched! Never experienced anything like it before or since, and of course, she knew what I knew and looked at me sheepishly. I whispered in her ear, "I want you to fuck him." She gasped and looked at me wide-eyed. I just smiled and nodded. She said, "Go, go, go" and pushed me out of the room.
She didn't come out for quite a while, but when she did, my heart started pounding furiously. You could really see everything and the fact that she was willing to wear that in front of him made me think it might really happen!
It took some time, but then they started dancing, and the clever girl had set it up so the tie holding the top on was loose. The bouncing of her dancing soon caused the top to come undone, exposing her big breasts. She just said "Oops!" and laughed, but took her sweet time about covering them back up. When she finally did and started tying her top back up, I moved behind her and offered to help. Then I said, "Actually, I think it looked better that way" and pulled her top back down, holding her in my arms to prevent her from covering up, although she made no effort to do so.
When she made no effort to cover up, a let her go and she continued to display her breasts, but nothing further happened. So I moved behind her and slowly pushed her towards him, and he took her breasts in his hands. She in turn reached down and started fondling his erection through his pants.
I pulled her wrap down and the rest of the way off, and now my sweet wife was fully naked in front of my friend, who lost no time in discovering as I had earlier, that her pussy was dripping wet. She pulled open his pants and dropped to her knees, taking his pants down to the floor, and began to suck his cock hungrily, looking up into his eyes as she inhaled his dick.
Before long, she lay back onto the big sheepskin rug in our living room and spread her legs wide for him, holding her pussy lips open invitingly. He moved between her legs, and I remember thinking for a while that "It's really going to happen" until at some point, my fogged up brain tumbled to the reality that it had already happened, and that he was pumping in and out of my wife.
It was such a hot scene, and I'll never forget it. We had some other encounters, but there's nothing like that first time, watching another man plunge into your wife.