Source, in full
This was said by Vidal's maternal grandfather, Thomas Pryor Gore, as recalled by Vidal: "My grandfather, Senator Gore ('I never give advice') was suddenly Polonius; he also changed his usual line from 'Never have children, only grandchildren' to 'Be not fruitful, do not multiply.' " [Palimpsest, ch. 3: The Desire and the Successful Pursuit of the Whole]