Answer:
He didn't really betray her.
She was always far more into him than he was into her, because Venus (Aphrodite) asked Cupid to take the guise of Aeneas' son and basically form an attachment between them, you will remember. So when they spent the night in that cave, she thought they were married, while he never really took it as seriously.
Later, when Jupiter (Zeus) sends down Mercury (Hermes) to tell Aeneas to go away and keep searching for his new land, Aeneas plans to leave without telling her (try searching for 'stoicism', and you will understand what was going through his head). She is understandably upset about this. They fight, he tells her they were never married, and he leaves.
She feels betrayed because she feels he is walking out on their marriage, their love.