I just listened to the full audio of The Book of Adam and Eve, very interesting although it's not biblical canon. It says that after their fall, Satan temped them almost daily and one of the forms he came in was that of a mermaid in order to tempt Adam into marrying eve.
Before then, they were not married and Adam being fooled by Satan made a promise with him to marry eve, but then God told him he was fooled and he asked forgiveness but still had to fulfill the promise so his animal desire from the flesh didn't consume him.