Mother can`t resist - Short story

"Peter, are you getting a hardon? Is that an erection I see in your shorts?" mother wants to know. "I am your mother and just holding you in my arms, caressing you a little bit."
Her son looks ashamed to the ground. I can`t help it he says with a quiet voice. He already thinks his mother will get angry and punish him for that.
But no! The woman slides a hand in the boys shorts. Wow, she thinks. That feels really great. It has been a very long time that she felt something like that. On one hand she thinks it`s wrong what she is about to do. On the other hand she can`t resist. Mother rubs the bulge against Peters underpants. Faster and faster. Mother gets so excited that she doesn`t care about what she is doing.
Peter wonders what is happening. What is his mother doing? He can`t believe it. For a while he is able to behave himself but then it happens. The boy gets an orgasm. His cock unloads and it seems it doesn`t want to stop. One load of sperm after the other is shooting out. The underpants get very wet.
Of course mother feels what happened and she smiles. "Don`t worry", she says. "It is OK."
At the same time the woman feels her pussy moistening. This can`t be, he is my son, she thinks.