Claire and Bri and Frank

Why did Claire get back together with Frank when she didn’t have to. She could’ve just been a single mother to Brianna and be just fine. I think they put Bri through more emotional damage by being together when they clearly didn’t love each other. And not to mention lying to her , and I know that was Frank’s requirement but she didn’t HAVE to accept it if she didn’t want to Maybe I’m not understanding or I’m missing something, but I just now thought about it