I had this same fantasy but couldn't set it up. So here's what I did that was almost as satisfying:
Got drunk one night with my wife and a friend who has always wanted to fuck her. She had too much and went to lie down, I knew she'd soon be completely out for the evening. We hung out for a while and then I went to check on her. After making sure she didn't stir at all I removed her panties and touched/tickled her between her legs, then I put a finger in. She still didn't move.
Went out and made a proposal to the friend, he said he was down so I brought him in to the bedroom. I opened my wife's legs for him and let him caress her, then he fingered her gently for a bit. He told me he was getting hard so I told him to take his cock out. He stroked it in front of me for a bit while he touched her, then I told him to go down and taste my wife's pussy.
As soon as he began to, I went down and took his cock in my mouth. I sucked him while he spread and licked her slit. He was so excited and I could tell he wanted to be inside her bad so I worked his head and licked all the way down to his shaved nutsack. It only took me doing this a couple times before he was ready. I really thought for a minute about letting him slide it inside her to finish just so I could watch his sperm spill out of her cunt. But before I even knew it he was jizzing deep into my mouth. It all went down, there wasn't even a chance for me to swallow. He told me it was the hardest he's ever cum from having his dick in someone's mouth. He confessed that he was imagining it inside my wife when he let himself jizz.
