You were raised by a single  mom. She gave up dating and suppresses her sexual desires to focus on you. Your bully takes advantage of her loneliness, plays with her for a while, then throws her away.

However, they released all her repressed desires, and when bullies leave, she becomes addicted to sex and has to get her fix from somewhere. You are the nearest man around, and so she makes you have sex with her. You witness your mom transform from a saint to a total whore.