How does it make Mary Jane Watson a liar? Both Norman and Gwen are dead shortly after the Sins Past flashback.
She didn't know what became of the children or that Osborn survived the piercing of his heart. If she told Peter without evidence, she'd worry he'd think her a liar. Peter would have written it off as Osborn using psychoactive drugs on her. Harry would never have accepted his father had a fling with Gwen Stacy and possibly murdered his newborn brother and sister. Informing either man would have only strained the friendship between the three people. MJ may have believed that Peter would resent Harry in his father's absence. Later, Norman Osborn returns and commits Infanticide in retaliation for Harry's death, Osborn's "only son". A grieving mother would assume the villain who murdered her child either put his own up for adoption or, more likely, did the same to Gwen Stacy's babies.
What purpose does telling Peter Parker anything after so long serve? It would give him more incentive to attack Osborn again in front of security cameras and further validate the villain's smear campaign against Spider-man. Or give Osborn what he wanted when the Green Goblin attempted to commit suicide-by-superhero because he couldn't deal with his issues anymore. There wasn't a time to deal with that situation, and all signs pointed to Harry as the only child to survive to adulthood.
...Or that no other offspring existed before Sins Past.
Hooking up with a legal adult is socially frowned upon for age reasons but was entirely lawful. Osborn was already a sexist murderer of children.
Gwen Stacy did nothing wrong and the story was at odds with 616 canon before Editorial touched it.