That line may have hurt them the most, it would have almost been better to kill them than emotionally traumatize them like this, Scott's character basically taught them hatred towards all men. That was an unnecessary way how to let them go, why not teach them about values of letting go in order to grow on their own.
shareJake said all men were bad so that Fa and Be would not trust anyone. Jake had to send a brutal message that a break had occurred in their relationship and could no longer be trusted. But Fa still knew he and Be were loved nonetheless and accepted this "break" in the bond that was once shared between his "parents" and him. His love moved him to make a few final attempts to communicate with them and his love finally moved him to stop and leave. Also, remember that after Jake tells Fa that if he and Be left "men would not hurt Ma and Pa". At that point, Fa understood that he and Be must leave. I believe that is why he finally stopped trying to reach them and left. He loved them and did what he was told to keep them safe.
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