I think it has always been in me to be this way. My first real girlfriend, the one I lost my virginity to, cheated on me with a good friend. When I found out instead of being hurt angry or feeling betrayed I just got excited.
I knew she was seeing him on the side while still having sex with me. Every time I got together with her I would bury my face between her legs and imagine that his dick had just been in there. My fantasy was to watch him fuck her then lick her to orgasm afterwards while his sperm was still there all warm and fresh.
One night I knew he was going over there. She would leave her bedroom window open for me to crawl in so we could fuck so I knew she was also leaving it open for him. I waited in the bushes until he showed up and crawled in. I gave them a few minutes and stuck my head in the window.
He was on top of her going at it.
I could hear him grunting away as he plowed her and the sounds she was making told me she was enjoying him a lot more than she enjoyed me. I got so hard and excited I had to pull my dick out and pull on it. I heard him grunt one last time as he finished and then heard her orgasm like I had never heard before.
I got off just then shooting it all over the side of the house, my head was in the window and I was gasping. He turned and saw me and said what the fuck as he fell off her. I can still see the look of shock on her face. I pulled my pants up and ran away.
They never knew that I liked seeing them fuck, they thought I had just caught them cheating on me. I was ashamed of how I felt or I would have told him I had no problem with sharing her with him. She thought I was angry so she avoided me and I was not mature enough or comfortable enough to tell her the truth.
FUCK MY WIFE!
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