Top Five Most Likely Reason Margie's Mom "Just Forgot" About Her Son's Request for Sex:
5) Realization that her son is a psychopath made her unwilling to provoke him by discussing the incident for fear he would rape her.
4) Did not mention again to him, but has since mentioned it to any number of people, including local police and mental health representatives
3) Convinced herself that her son's request to "provide sex" for him was in fact a request that she provide him with a prostitute
2) Margie is a sick, stupid limpdick who made this story up in the misguided belief it would make him look like the alpha-male rape apes whose dicks he so desperately longs to suck. When the story was not received with praise for his heroism, he started backing off it; and...
1) Mom's actual reaction was to thrash his ass, and he's too embarrassed to admit it.