"When I accidentally found her secret Yahoo IM name. MILF4U. That led me to check the chat logs, and that led to a divorce."

Some submissions are also from this Reddit roundup because they're just too good not to share.

"#2 My girlfriend was home from college for Christmas. She said she was going to go spend the night at her parents’ house. I was bored, so I decided to go to a movie I wanted to see. It was sci-fi, so I knew she wouldn’t want to see it. I went in, took a seat in the middle of the theater. As the lights dimmed right before the movie began, a couple hurried in and sat down in front of me. It was my girlfriend with a guy I recognized from her college. They were holding hands and being quite intimate. As the movie ended, I moved to a different seat, then followed them. They walked a few blocks to her father’s store. It was late, so it was closed. She unlocked the door, and they went in. The office lights upstairs turned on. I walked up a few levels in the parking garage across the street, where I had a lovely view of my soon-to-be ex-girlfriend, on the couch in the office, naked and enthusiastically riding her college friend. Adios!

#3 My live-in girlfriend came home in the morning after spending the previous day and night with a friend. She had a thin duffel bag for her clothes. She tossed it on the couch. When I sat down beside it, I felt a hard lump in it. I could even see its shape through the thin material of the duffel bag. Looked familiar. Reached in, pulled it out. Yep. Her diaphragm case. Opened it. Still messy and obviously recently used. Closed the case, and the case is closed. Goodbye.

#4 Living with my fiancé. She went out with her friends. Came home at 4 a.m. Immediately showered, which seemed a bit odd. Crawled into bed. I decided to make love. She seemed less than enthusiastic, but I proceeded anyway. Now, let me just say that from prior experiences, I was familiar with what 'sloppy seconds' feels like, and I was feeling it. Hard to explain exactly, but it’s definitely different. Less slippery, more slimy. Even a bit sticky. Thought maybe it was my imagination, but this was back before cell phones, so I checked the phone bill, and there were dozens of long-distance calls to a town 40 miles away. Dialed the number. Guy answered. 'Hello, Bill speaking.' Goodbye fiancé.

#5 New fiancé. Living together. She went to visit relatives in Tennessee for a week. Returned home and dumped her dirty clothes into the laundry basket. Her fanciest black barely-there lace panties were on top of the pile. And, well, let’s just say an impressive cream pie was quite evident in the crotch. I probed a little deeper and found three more pairs of especially sexy panties equally despoiled. Despite the odd choice of taking her sexiest panties for a trip to visit family in Tennessee, giving her the benefit of the doubt, I thought perhaps she had some sort of yeast infection. I happened to have a very powerful UV light for my work. Shone it on the laundry basket. Male cum glows under black light. Her panties lit up along with several stains on her jeans, even a bright blob on one sock. Guess she stepped in it. She must have had a lot of catching up to do with friends in Tennessee. Sent her packing.

There’s a #6 and #7, but I’m out of time."

"The next day, I demand the truth. She tells me her work asked her to pick something up, she invited a 'friend,' and 'nothing happened.' Yeah. Right. The first time she kissed someone else years ago, or when she told people we were in an open relationship (we're not), I should have been done. At this point. I just want her to acknowledge the truth. I know I’ll never get it. I’m devastated. I put far too much of my soul and life into her hands, and she crushed it."

"I'm still pretty bitter about it, but I shouldn't be. The STI is gone, and it's kinda funny how science so simply confirmed gut feelings that I'd had for months."

"When my wife finally came home, she made some shit excuse. But I confronted her, and she confessed. She was cheating for two years behind my back. Even had the guy over while I was gone to work. I've been single for four years now. Happy as ever. Daughters live with me. Ex-wife is still with the same guy. Four years after our marriage. Two years into our marriage. So they have had a good run."

"Fast forward to the next day, at the funeral home. She is being distant and basically ignoring and avoiding me, but every time I find her, she is with this guy that is supposedly her cousin. I chalk it up to her being upset; she was close to her aunt. I go out to my car to grab something from the glove box, and there I find her, giving her 'cousin' head in the backseat of my car. So, not only did she cheat on me, she cheated on me with someone she told me was her cousin, in the parking lot of the funeral home, at her aunt's funeral, in the backseat of my car."

Responses have been edited for length/clarity.