You can't be just a little bit pregnant
It means that the situation that is being described is a binary. It is something that is either IS or ISN'T.
Such as is the case when one is pregnant. One either "is" pregnant or "is not", there is no in between or partial pregnancy. It means it is a situation in which there is no
ambiguity or no
gray area. You can't be "just a little bit" of certain things.