What does the joke can you read,can you write,can you do it all night means?

It goes like a guy asks a girl and says Can you read? And she nods yes and then the boy asks can you write and the girl says yes and then the boy says can you do it all night and she says yes and the boy laughs and says so you'd have sex all night?
I guess I'll never get that joke
Please explain

if you say can you read you can and when you say you can right you can but if you say you can do it all night you are gonna get a baby

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Wow... No wonder little me didnt get it, im pretty sure the boys who asked me this liked me. Thanks guys.

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