It is not sinister in the same capacity as Seema or Supriya's betrayal and yes I would not even call it a betrayal. Up until now her decision to withhold the truth was justified. She is not guilty of forsaking his love but of not trusting him as a friend/husband. The secret will be damaging to his impression of Naku as an honest, righteous person. Her intentions were pure but she'll still have to face the repercussions for her delay.
At the same time, her secret is not a cardinal sin. It does not warrant punishment like a betrayal would. He will need time to adjust after feeling disconnected from the fair Naku and he has every right to feel hurt. However, it would be wrong for him to oversimplify the issue by associating it with his beauty = betrayal theory.