Okay, the full story:
It was several years ago, can't think exactly how many, but I reckon about twelve.
Anyway, we were invited to a big party - an 80s themed party (80s music etc) - and all our friends would be going. Trouble was, I had not long had a knee operation (ACL from playing rugby) and I don't much like 80s music anyway so I decided to stay at home. I was still in plaster after all. I knew that my wife had been flirting a lot with one of our friends and they clearly got on well (he's married also, by the way). I know he fancied the hell out if her. He had not hidden that and had even told me that she had the best tits he had ever seen (he'd copped a few looks down her top over the time we've known him) and even joked once that he would love to cum on them.
I helped my wife get ready to go out. She wanted to look the part. They were all going in 80s clothing etc. This meant that I knew exactly what she was wearing. Anyway, she looked great and off she went.
Later in the evening I got a text from a number I didn't recognise. It had a photo on it. My wife and my friend were kissing and it looked like a proper full on kiss too. Obviously, I was suspicious but went to bed at about midnight. Then, at around 3 am I heard the front door open and close. She was home. She went into the down stairs toilet and was there for ages. I was even going to go and see if she was alright, but then heard her coming up the stairs. I stayed in bed in the dark and kept still so she'd think I was asleep. First of all, I saw her throw something into the laundry basket. Then I watched her undress. She took off her top and she wasn't wearing a bra. She definitely had one on when she went out. Then, she took off her jeans and I was really very surprised to see she was no longer wearing any knickers. Again, she was definitely wearing some when she went out. It was obvious to me that this was what she had thrown into the laundry bin. Even at this point I was starting to feel aroused because I had put two and two together. I reckon, she had spent so long in the downstairs loo trying to let all his cum run out of her pussy, but there was probably so much already in her knickers that she couldn't put them back on.
She got into bed and went to ....... I lay there not knowing if I was upset or just turned on. My mind was racing, but it soon became obvious to me (because I had an enormous erection) that I was seriously aroused by the thought of this.
In the morning, she said she had to go out urgently. She didn't tell me where. But I now reckon she needed to get to a pharmacist to get a morning after pill. So she left. I got up and went straight to the laundry bin where I found her knickers from the night before and the gusset was full of a huge amount of cum. I know it could be a lot of things but it looked like cum to me. I looked at her other knickers in the bin to see if any others looked remotely the same but none did. It was cum. I'm certain of it.
I was a bit worried. I didn't want to lose her, but I didn't think that was likely to happen. Within a few minutes I was masturbating whilst thinking about what she had done. I think I now know where they went to do it too. I often masturbate thinking about all of this even now.
About a week later, my friend came round to our house for a lads event watching rugby. My wife was quick to get up and answer the door to him (she hadn't been so anxious to do so for the other guys) and I could tell there was no more flirting and it was a little awkward between them.
That's about it. But I reckon it's pretty certain that they fucked. I've never confronted either of them about it. I realised from that moment on that I was turned on by the thought of her fucking someone else. It also made me realise that she was willing to fuck another guy.