FanfictionAddict

Reference:

why are u the one asking...?

It makes sense that Gero was leaning towards accepting Tonako's proposal before Kinosaki's interference. In general, people are drawn to and feel safe with what is familiar to them. Gero was raised brutally to become the head of the Poison Users. His life was controlled by his grandmother and her underlings. The kindness he received from the girls he met with Kinosaki felt too bright (unfamiliar) to him. He didn't believe he could truly reciprocate in a relationship with any of them. Gero was afraid of the unknown of a relationship where he gets to make choices for himself. Any other route than Tonako would lead to mistakes and hurt feelings as Gero learns to be his own person.

Tonako offered a relationship with the most familiar dynamic to Gero's upbringing: a relationship where he has no agency and is valued for his bloodline and brutality/skill as a Poison User. Gero understands exactly how to fulfill that role without having to experience any further character development.