This is a topic for another thread (and there have been many, many threads on this topic), but consider this; if there were other blood children of Mary and Joseph; Joseph is no longer around (presumed dead) at the time of Christ's ministry); why were they not there to care for Mary at the time of the crucifixion? Without a husband and without blood male children, a widow would have been destitute. In caring love for His mother, he charges John with her care; caring for her as a son would care for his widowed mother.
Now, perpetual virginity aside, this makes a very strong argument for no blood siblings; step brothers and sisters maybe, but it would seem to indicate that Mary had no more children. This being the case, the PV is not that much of a stretch.
Also, as you noted, while not dogma in every Church, the numerical percentage of Christians world wide do accept the PVM.