When they count the Protestant vote, they count the lesbian preachers, blacks, people that the Baptist churches have purged from their rolls, Hispanics, Episcopalians, people that don't attend any church, the Reverend Wright church, everyone that says that they are Christians that aren't Catholics.
Your comparison is a false one. When a Protestant leaves his denomination he does not continue to identify as a member of that denomination. Catholics who leave or who stay even when they don’t believe continue to self identify as Catholic. The exception is Catholics who become Protestants. But ask somebody who was raised Catholic but who has not set foot in a Church for 15 years what religion they are and they will say Catholic. Even thought at that point it is a meaningless definition.
Just as there are secular Jews there are secular Catholics. If you polled such secular Jews you would find they supported all the issues that Orthodox faithful Jews oppose. Such as homosexual unions.