Because they are cults, like Rome. The RCC's denial of marriage by the "priesthood", canonization of "S"aints, sacerdotalism in general, papalism, indulgences, purgatory, sacraments for salvation, and the other 400 creepy traditions foisted on the poor unsuspecting public are just as ungodly as an organization promoting homosexuality. It is clear evidence of their error and disconnection from biblical truth.
Then how exactly does one distinguish true protestants from fake ones?