You hit the nail on the head here. The belief that Mary had no other children is foundational to RCC theology, if it is false their theology crumbles to dust ... ours could care less.
sorry, no win. It's not "foundational" as is the foundation stone that Jesus Christ is God incarnate who died for our sins and who rose from the dead to win us eternal life.
"yours" as in which one? Is this the same theology that says that "homosexual discrimination is a thing of the past, let's have gay marriages as that is the message of Jesus"?