Palin's decision about her 5th child is a personal issue. Should it be part of politics? probably not, but when you are in politics your personal life is not personal, it is public.
For instance, think about Obamas personal life, (no relationship with his father, being raised by his grandparents) people don't talk about how "disgusting" that is. Obama said last night that the absence of his relationship, or lack of one, with his father makes him stronger. Well, IMO, the challenges on dealing with this 5th child will make her stronger.