I suppose it would be a problem to the modern secularist protestant view that extols the supremacy of individual above God.
Like I said I'll leave the quantifying to the statistical experts. I guess some are just itching to perform their Glenn Beck chalkboard flow-chart act.
There is no such thing as a secular protestant, in that Protestants cannot be born into their religion; if they are born again, they accept Christ's teachings. If not, they are neither Protestant nor Christian, regardless of what they say. Same is true for Catholics, by the way.
The only major American religious group who can claim secularism along with the word that indicates the religion of their fore parents are those who are ethnically Jewish but do not practice Judaism, since there is a racial characteristic to being recognized as a Jew.