It happened, while they were there, that the day had come that she should give birth.
And while it is true that, in a technical sense, "firstborn" does not prove there were other births, it certainly does not deny the fact; and, coupled with the repeated mention of Jesus' "brethren" in the Gospels, it is conclusive.  Allegations to the contrary are founded upon a mistaken premise that the state of virginity is holier than the state of matrimony, declared by an apostle to be: "honorable in all."