After years of talking about it and both agreeing that we "would" if the opportunity arose, my wife finally had a friend of mine. It was super hot, and she and I wanted to do it again. So we invited him over for pizza. I told her that I'd go pick up the pizza and she should try to seduce him while I was gone. I told her to turn the porch light on after she either succeeded or gave up trying and that I wouldn't come back until I saw the signal.
Rather than go get the pizza, I parked down the street and walked back, where I was able to see everything that was going on inside through the big picture window in the front room. She had brought out a photo album and was showing him pictures, with him seated, and her standing next to him, her arm draped around the back of his neck, her big boobs on display at eye level, bra-less with her erect nipples clearly visible through her blouse.
Especially given that they had already fucked, her flirtation was unmistakable. Before long his arm was around her too, and then fondling her ass, and sliding up under the back of her blouse. She responded by pulling up her blouse and pushing her tits into his eager mouth and hands. Soon, she dropped to her knees, and although I couldn't see because his back was too me, there was no doubt that she was sucking him off. Within a few minutes I could hear him grunting as he emptied into her mouth (she always swallowed everything).
I raced to go get the pizza and returned to find the porch light on. When I went inside she was beaming and motioned to me down the hallway where she proudly whispered "I sucked his cock!" I never did let either of them know that I watched.