My rationalization is rational, at least. They are good reasons. Let me expand upon the "sibling rivalry" point. Mary remained without sin her entire life. She was the new ark of the covenant, a perfect vessel. One would not even think of desecrating the holy vessel which held God Incarnate with mere human contents. That's the spiritual significance.
SD
Not to mention the fact that counter arguments abound based on logic that would be equally valid without in any way risking the purity of the Lord. Ie. God, knowing that Jesus was going to die, would not leave a family without Children. And the argument of sibling rivalry looses more ground to "Why can't you be more like Joseph's son". This stuff happens in every day life without divinity. The sibling argument denies reality.
As far as the "Are these not His sisters" statement. They are precisely posing the question that if these all came from the same parents, where did this One, Jesus, derive all these things the others do not have. This flatly loses it's meaning if one trys playing word games (that do not fit btw). It is intended to reflect his divinity.
It is my personal opinion that the beliefs at the time this doctrine was "developed" were that sex was dirty and Jesus mother couldn't have sullied herself in such a disgraceful act as to have sexual intercourse ever (even with her lawful husband). Of course, once the doctrine is developed it's not likely to ever be changed. It's not in their way of doing things to admit they were wrong.