The only way I could justify is that Virat is emotionally unstable right now with losing pakhi, with Kamal's death, and with Samrat being MIA. And with the villagers pressuring him, he is not able to think clearly. Now that Sai had committed suicide, he is now floundered by what he should do. So he just gave in to the villager's pressure. From what I witnessed in Virat's character is that he tries to please everyone. In the first episode, you will see how he goes to the temple to keep his promise to his aunt, he sacrifices Pakhi for his brother, he then makes that stupid promise to Pakhi, and finally, he makes his final promise to Kamal sir. So essentially he does what would please others in the name of duty. I guess as a police officer he is not very great at separating his personal emotion from his duty. That is the main character complex with Virat.
For Sai, I think she would agree to Virat because she knows subconsciously that she will be a burden to these villagers. Then when Viral strikes the deal with her she would feel more at ease to accept it as the marriage would not really mean anything to either of them. This would give her a chance to stand up on her own feet and give her the leverage to leave when she wishes.
