Hugh Hefner’s widow Crystal wasn’t allowed to go downstairs to the parties at the Playboy Mansion without him. The former Playmate, 37, was 21 when she was invited to stay at the Playboy Mansion in October 2008, and a few months later she and the late Playboy mogul entered a relationship – before Crystal became the pornographer’s third wife when they tied the knot in 2012. And she has given her honest perspective about her time with Hugh in the mansion in the memoir ‘Only Say Good Things: Surviving Playboy and Finding Myself’. While promoting the tell-all tome, Crystal revealed one of the rule…