Frank and Judy have worked together on numerous films and have allegedly had two romantic encounters. One was whilst she was still married to her husband at the time, Vincent Minelli, and the second, when she was separating from husband Sid Luft.
Marilyn was another actress on the list with an extensive list of lovers. It has been rumored that Frank tried to help her with her drug problem and that he even asked Monroe to marry him at one point.
Besides being married four times each and both being legends, Frank and Joan have never confirmed nor denied that their relationship ever existed. The rumors may have come about because they were spotted out together on numerous occassions.