I shared my first wife for about seven years. While it lasted, it was hot, but also lots of work. My ex and I fucked every day and several times a day on the weekend and vacations. We often went months and months, however, without another man penetrating her, but the potential was always there, so we could fantasize about it. In addition, each time I inserted my cock into her, there was a small chance that I hadn't been the first to explore her vagina that day, which made it extra exciting as I probed her pussy for evidence. I was only surprised a few times.
The first time was on a Saturday afternoon. I had been to the flea market with a buddy early in the morning, while she slept in. When I returned at about 1pm, we stripped and jumped in bed, making up for our not fucking in the morning. After no more than three strokes in and out of her, I asked, "Hey, am I the first one here today?"
She broke out laughing, "No, you're not. I wondered if you could tell. Didn't you notice that the bed-sheets had been changed?"
No, I didn't notice the sheets, but her pussy felt looser and wetter in a sticky way. She told me that while I was gone she had rushed out to the corner bookstore to buy a new novel and she ran into one of our special male friends. He was buying a map for an upcoming trip and he went back with her to our place for a quickie, before he and his family drove off on a trip. He was extra horny, so the quickie took closer to 45 minutes to complete.
As she described the event, my cock swelled and throbbed inside her, bathed in another man's semen. I kissed her deeply and it was there, the faint smell and taste of another man's cock. I then ate her and her intimate scent also betrayed his semen in her and her desire for his load. (Once precum or cum mixes with her own sexual fluids, the resulting smell was unique.)
I cannot think of a nicer surprise.