I remember what got me into this. I was around 14 or so at the time. I used to go snoop around in my moms room when nobody was home. One day I was looking in drawers and found her diary. I starting reading through some of the pages, and stopped on a page where she had written about this guy she had sex with, she wrote about how great the sex was. I was in shock at the thought of my "innocent" mom having sex. Her and my dad had been divorced ever since I was little, so I never thought of her having sex anymore. She never brought any guys home, but used to go out every Friday night, I just assumed it was with friends, and I didnt care cause I always got the house to myself. After reading that in her diary I could hardly look at her for a while. I never mentioned anything about it, and eventually I got over it. Fast forward a few years later, and I was jerking off one day, and the thought just popped into my head of watching my mom have sex. I got so hard, and came so much thinking of that, that ever since Ive had this fetish. I even went back years later to see if she had a diary again, but Ive never found anything since. But nowdays I enjoy looking at pictures of her, and imagining what it would be like to watch her get fucked.
Edit: I also remembered something else that I seen when I younger that I really liked. We had a treadmill in the living room so you would walk on it while watching tv. My mom would use it some days after work. I remember one day she was on it watching tv, and I went and sat down on the couch which was behind it. And before I know it, my eyes are starring at my moms butt walking on that treadmill. She was wearing really tight jeans, and my eyes were hypnotised as her butt went back and forth. I got so hard, I had run to run to my room and jerk off, I remember cumming so much.