With my ex, it was the one and only time during our marriage that she let me share her. She had actually initiated the event when it happened, after years of me telling her about my fantasy and us both getting off to it. She let a guy think he was picking her up (while she picked him up!), and then told him she wanted to go back to his hotel room with him, along with her husband. He was all-in. It came as no shock to him, since I was there with her, watching them get increasingly frisky (it turned out, he'd been trying to get his wife to do the same for years, but hadn't yet succeeded).
After a very long night of fun, in his hotel room, we were getting ready to leave,. He said he wanted to fuck her one last time, so he did, and shot another load into her (must have been somewhere between 6-10 loads in her by now over the course of the night). I had often talked during our fantasy play about eating another man's cum out of her, but I'm sure she never thought I would. And actually, when faced with the prospect, I wasn't much into it at that moment myself.
However, it occurred to me that I might not ever get another chance, and also that my fantasy talk about it in the future would ring hollow if I didn't do it now, so when he pulled out, I moved in and began to gingerly tongue at her. Flicking just the tip of my tongue across her clit and not really getting any of the pie at all, but just to say that I had.
She was at first quite surprised and it took her a while to believe it, but when she saw that I was really doing it, she got wild (again!) and began to buck and moan. This turned me on so much, it prompted me to stick with it, and very soon I was licking her deeply, going after every last drop. She came harder than she had ever cum before, which was saying something after that night of fun we'd just had!
When we got home, I went down on her again and made her cum again and then fucked her again and dropped another load in her before we fell asleep exhausted. I woke up in an hour or so with a raging hard-on at the memory of what we had done, and went straight down on her as she slept, then fucked us both to another orgasm. By now, it was pretty much just my own pie I was licking, but it triggered my awareness of what we had done, and how another man's cock and sperm had so recently been in her married pussy. This repeated every 2-3 hours or so until she told me she was just too sore to continue!
My forever wife, the woman I am married to now and for the last 17 years, has come home a few times with a nice pie (she only plays alone). She will grab my head and push it to her pussy and tell me to eat it, telling me what it is and how good his fuck was as I comply.