Moms Secret

I was 15 when I caught my mother cheating on my dad. School ended early that day and when I arrived home mom was nowhere to be found. I went upstairs to my room, but as I passed my parents' bedroom I could hear a groan. I looked through the keyhole and saw my mother, naked on her bed on all fours. There were two other people there. One behind my mother, the other in front of her and both penetrating her.
I watched for a moment and couldn't believe what I was seeing. The two others were boys from the neighborhood. One 17, the other 19 years old. I could see them taking my mom hard and she was enjoying it. They slapped her butt, called her bitch and both finally came inside her.
I never told her that I saw them. Nor did I ever tell her that I jerked off right at the door.