The worse I've ever felt in a long time was when I was 19 years old, and I slept with a woman during a fraternity/sorority exchange, only to find out that she was married to an enlisted Marine serving in Iraq.
Our meeting starting simple enough: We were both getting drunk and started making out on the dance floor and ran off to some back room and started hooking up some more. She started blowing me and then we all went back home to our fraternity house where I pounded her out. It wasn't bad, she had really great t*ts.
Anyway, we texted back and forth for a while. It was cool, we hooked up a couple more times. Then all of a sudden, a couple months after our initial hookup, I got this text from her phone: "Hey, how many times did we have sex?" Thinking it was weird, I asked, "Why do you ask? You were there." A while later I then got a call from a crying, angry Marine telling me that I slept with his wife and that I had no right to violate the sanctity of their marriage and that I was a terrible person. I let him vent. I didn't know that he was her husband or that she was even married, so I let him say his piece because he was obviously hurt, and with good reason to be hurt.
I explained that I didn't know that he existed and that he could do better and that he should dump her and never speak to her again. He accepted my apology and I tried to introduce him to other girls. He appreciated the effort but wasn't feeling it because he was still torn up over his now ex-wife. I still sometimes find it mildly amusing that of all people to tell him that he was too good for her and that he should leave her, it was the guy who she cheated with.
I called her out on that. She started crying and said that she was really confused and didn't know what she wanted from a guy when she cheated on him but that she was going to church now and that she was taking steps to make things right. I immediately wondered how many dudes at that church were plowing that.
Anyway, I am certain my story isn't unique. It felt shytty being the other guy because in that Marine's mind he was the gallant warrior with a beautiful woman waiting for him at home while he was off saving the world. Instead he got cheated on, wounded, and nearly forgotten by the people that sent him to fight.
It's not easy to be a man, sometimes. It's also why being married seems like such a foolish thing to do. I don't see why I need a woman in my life when I can take care of myself just as easily.