My wife (fiancée at the time) was cheating on me with one of my college buds. I was confused. I felt jealousy, but the arousal was overwhelming.
He and I played intramurals together. In the showers, I noticed he and I were both lean and ripped, but his flaccid cock was probably bigger than my erect cock. I found myself masturbating to the thought of him fucking my fiancée with that enormous cock of his.
I played dumb to the obvious clues. Even when I would go down on her and I could taste his cum. I started doing things to make it easy for her to keep fucking him as much as possible.
She eventually confessed, but was not expecting the response I gave her. I told her I didn't want to lose her, but I confessed that I had known, all along, and that I was aroused by her having sex with him. It blew her away, even more, when I told her that as long as she loved me and wanted to marry me, she was welcome to keep having as much sex with him as she wanted.
The two confessions definitely shook our relationship, but we kept talking. She realized that if I was aroused by it, we were both getting something out of her having sex with him. She started having sex with him again and, of course, he was shocked that I was ok with it. Apologies were made for the deception.
It took a while, but the three of us finally had a threesome. Then another, then another, etc.
He ended up moving in with us. The three of us shared one bed. It was a blast. We became a poly threesome, but I was aroused, the most, just watching them have sex. He didn't just have a bigger cock than me, he would fuck her tiny body with a violence I simply could not duplicate. When they had sex, she would make noises she never made with me and he would make her entire body tremble and shake.
I had no problem submitting to the better man. I wanted the sex she was having with him to be her everyday sex. So, he became her primary sex partner. She would give me blow jobs while he fucked her, but she and I were having actual sex once, maybe twice, a month. Even though they were having sex several times a day.
It was crazy and I fully expected to lose her to him, eventually. But, I didn't. She still married me in the middle of our senior year, even though she was having the vast majority of her sex with him.
That set the course for our marriage over 30 years ago and we're still going strong.