Since she knows what tactics the enemies use, in particular Sadi's faction, she probably wants to make sure Hayato isn't the kind of man that is easily seduced or tempted. The future of the world is at stake and she understands the importance of having someone to rely on, she had Leo and ultimately it was thanks to him that Eviru was defeated and they foiled Kurone's plan. She knows Celia can't fulfill her role as the hero alone, and having a weak partner will only make things harder for her. Also there's a chance that Luna is aware of the relationship between Hayato and Meryl, so her opinion of him is probably pretty bad.
