I suspected my wife (fiancée at the time) was fucking my best buddy. We were in college and living together. My best bud's apartment was right around the corner from ours.
The two of them took a class together and they would study together at his apartment. She would come home, after studying with him, and immediately take a shower.
One night, while she was at his apartment, I decided to walk over there. I peeked in his bedroom window and sure enough, there she was riding his huge cock. I knew I was supposed to be exploding with jealousy, but instead, I was extremely turned on by what I was seeing. I wanted to keep watching, but I was afraid someone would see me looking in his window.
I didn't want to lose her to him, but I was so turned on by the thought of her fucking him, I continued to play dumb to all of the clues. I was even doing things to make it as easy for her to fuck him as much as she wanted.
Their affair got exposed, eventually, and I thought she and I were done. But, I took that opportunity to make some of my own confessions. She couldn't believe it when I told her I was okay with her fucking him, as much as she wanted, as long as she still wanted to marry me.
He ended up moving in with us and we became a poly threesome. He was my best man at our wedding and several of her girl friends knew that although she was marrying me, she had the majority of her sex with him, lol.