Given her love of self-righteously bleating "But that's DIFFERENT!!" (when it actually isn't) when confronted with the fact that her kids are doing to her what she did to Marian tells us that it's one thing for him to hide things from her and quite another to hide things from her mother. She assumes for good or ill that Marian is a punitive monster who expected her to have no social life whatsoever and who would throw her out on her ass and disown her and a whole lot of other horrible crap like that. This is because she thinks that the G-rated pursuit of a paperboy is actually a scandalous tale of wanton lust that no one must ever know about....especially the cruel monster mother from the olden days who hated young like like hers because her life was over.
This, as Elly said, forced her to burn her diary lest her mother find it and cast her out whilst yelling "Fly, Lechery, FLY!!!" like she's insane or something. This is sort of stupid because my guess is that Marian read it anyway.....and couldn't make heads or tails of the poorly constructed mush that was either a declaration of love or a pledge to kill and skin some random boy off the street.