Humans are naturally wired to feel attraction.
Being in a committed relationship doesn't stop them from noticing others they find attractive.
Attraction can come from many things, like how someone looks, how they connect emotionally, or having common interests. But just because someone feels attraction doesn't mean they want to act on it or hurt their current relationship.
For a woman like Katherine, who deeply valued loyalty, feeling attracted to Vritra in the slightest while being married to Cedric caused her a lot of inner conflict.
She had been struggling with this uncomfortable feeling of attraction ever since Vritra stood up for her and protected her in the hall.
Sometimes, she would think about it logically.