This happened to me although not with a married woman. I was having a friends with benefits relationship with this woman i met when i was 18 and she was 20, we would meet up 2-3 times a week and just go crazy. We knew each other semi-well but i had never been to her home or met any of her family, whereas she met my mum (i introduced her as a friend though) and had stayed at my house a bunch of times.
Anyway about 6 months later i found out that she was engaged to a man in the British Army and he was serving in Afghanistan (this was circa 2004), and they had been together since they were 15 years old and planned on getting married after the tour he was currently on. I only met a few of her friends and they all hated my guts. One of them told me everything about their relationship and was amazed i wasn't aware of it, she showed me his Myspace page and MSN Messenger profile where they had pictures together and all sorts.
I ended things immediately. I saw her a year later with her friend that told me, and she had told me that she had broken up with her fiance (her friend confirmed it lol) and wondered if i wanted to get together some time but i noped out of that pretty quickly.
I hated myself for being that other guy but she played it so well. She didn't wear her engagement ring and largely kept me away from her family and the majority of her friends too.
