She was so interested in him that she had sex with him even though she thought he was married. If he had been married, they would have both been guilty of having an affair, and she could have kept on with it, if poisoned by mutual guilt. But he wasn't--so now she knows, that he knows, that she's someone who would have been willing to commit adultery to get the man she wanted. She is deeply ashamed of this fact and doesn't want to face him.