"Fail in sex !" joke

One day Johnnie goes up to his mother and asks:"Mom, how old are you?"
Mom: "That's a personal question. You don't ask those kinds of personal questions to women."
"How much do you weigh?"
Mom: "You're too young to understand that you don't ask those kind of questions to women."
"Why did Dad leave us?"
Mom: "You're too young to understand that too, I'll tell you when you're older"
So Johnnie goes back to school and tells his friend: "My mom doesn't want to tell me how old she is or what she weighs. She doesn't answer any of my questions"
His friend replies: "You should go into her purse and look at her driver's license. All your questions will be answerd.
Johnnie goes back home and look into his mom's purse and looks at her driver's license and goes to his mom:
"Mom, you're 39 years old."
Mom: "Yeah that's right I am."
"And you weigh 55 kg."
Mom: "Yupp that's right."
"One last thing... I know why dad left us."
Mom: "Oh really, why?"
"Because you got an F in sex"