covertigamous
When a married person, who appears to be loyal and sincere to his/her spouse, is actually having affairs with more than two women/men. He/She is so good at concealing the affairs that no one knows each one exists.
George is married to Anne, who doesn't know he has affairs with Kathy and Genie. Both women don't know he is married to Anne and has other women in his life as well. Now that's what you call a real covertigamous.