The source was "Fatal Passion" the only full length biography on Ducky. However I do got other independent sources confirming that it was a man. It does make sense as her argument for leaving Ernie was that he was gay and did not love her (as she wanted to be loved). Her hard fought battle end up with her husband with a man. That would truly kill her. Ducky's pride would not let her forgive the her husband, but she also couldn't forgive herself for making that mistake. As Missy wrote "She was the most unforgiving of us all". No it could not be Sandra since Ducky was the one sister that she respects and fears.