That is utterly ridiculous and absurd. Just try changing the "human fetus" in the liberal proposition variable to black man, Jew, elderly person, or woman and think about that long and hard.
Ridiculous maybe, but only because it is hard to believe that a person has never considered the arguments for valuing a fetus (or "black man, Jew", etc). It is definitely not absurd, however. It is absurd to require people to explain their lack of position on an issue. If you value a fetus (or "black man, Jew", etc.), then I would think you wouldn't have any difficulty explaining why, so you shouldn't consider your burden to heavy to bear. However, the burden is still on you, the positive claimant.
Perhaps you just chose the wrong quote. Why not choose a quote with a positive stance, like "women should have the right to abortion" or "arguments justifying the value of fetuses are irrational" or "human life begins after parturition"?
But one cannot require justification of the statement, "I don't value X", because such a statement is in itself a lack of justification.
Kerry's status as an American citizen does not absolve him of this duty as a Catholic.