Mine was when wife told me a guy at her job was always flirting with her and once showed her his cock. Rather then be mad it turned me on.
I asked her what she thought of him and she just laughed (as if she liked what she saw) as said he had a really large cock. Days later I confessed to her that it turned me on the idea of her with a large cock. It made our sex life more spicey as we talked about it. One day I get home early and when she gets home from her job, shows me her tits and she had small bite marks.
She laughed and told me she knew I wanted her to fuck him and she did. They drove to a rural park in the outskirts of town. He had an older van they she jumped into. He took her shirt off and fingeredher. (Her weak point) pulled out the biggest one she ever saw and well, proceeded to give her the best pounding she ever has.
She came twice and he just once because it was getting late. I was so excited I just took my wife on the bed and as I entered her I felt how much this guy left her open! Since that day I realized I am a cock and love it.
She started doing him twice weekly until he moved from the company. She now has a boyfriend and he sometimes lets me watch