Fu Huai'an already had a son, so he probably didn't even care whether Gu Hanyan could give him another child or not!
Fu Huai'an's requirement for marriage after the pregnancy was to keep Gu Hanyan trapped so that Gu Hanyan wouldn't have children. Unable to give up the Gu Family's 30% share to marry someone else, she could only accept her fate as the stepmother and caregiver of Fu Huai'an's child.
Gu Hanyan wasn't stupid. She could imagine that when the child grew up while she was unable to conceive Huai'an's children, Fu Huai'an could claim he married her with the generosity of his heart. Meanwhile, the agreement could be easily changed.
In the future, Gu Hanyan was concerned that even if she wanted to get a divorce, it would cost her family a terrible price.
Gu Hanyan went back to discuss things with her parents and was unwilling to get engaged to Fu Huai'an, but the agreement has already been signed—the Gu's 30% share was no joke.