Virginitas in partu

I’m going to briefly compare two Marian dogmas. Here’s the rationale for both:

Immaculate Conception

And indeed it was wholly fitting that so wonderful a mother should be ever resplendent with the glory of most sublime holiness and so completely free from all taint of original sin that she would triumph utterly over the ancient serpent. To her did the Father will to give his only-begotten Son -- the Son whom, equal to the Father and begotten by him, the Father loves from his heart -- and to give this Son in such a way that he would be the one and the same common Son of God the Father and of the Blessed Virgin Mary. It was she whom the Son himself chose to make his Mother and it was from her that the Holy Spirit willed and brought it about that he should be conceived and born from whom he himself proceeds.

http://www.papalencyclicals.net/Pius09/p9ineff.htm

Virginitas in Partu

I answer that, Without any doubt whatever we must assert that the Mother of Christ was a virgin even in His Birth: for the prophet says not only: "Behold a virgin shall conceive," but adds: "and shall bear a son." This indeed was befitting for three reasons. First, because this was in keeping with a property of Him whose Birth is in question, for He is the Word of God. For the word is not only conceived in the mind without corruption, but also proceeds from the mind without corruption. Wherefore in order to show that body to be the body of the very Word of God, it was fitting that it should be born of a virgin incorrupt. Whence in the sermon of the Council of Ephesus (quoted above) we read: "Whosoever brings forth mere flesh, ceases to be a virgin. But since she gave birth to the Word made flesh, God safeguarded her virginity so as to manifest His Word, by which Word He thus manifested Himself: for neither does our word, when brought forth, corrupt the mind; nor does God, the substantial Word, deigning to be born, destroy virginity."

Secondly, this is fitting as regards the effect of Christ's Incarnation: since He came for this purpose, that He might take away our corruption. Wherefore it is unfitting that in His Birth He should corrupt His Mother's virginity. Thus Augustine says in a sermon on the Nativity of Our Lord: "It was not right that He who came to heal corruption, should by His advent violate integrity."

Thirdly, it was fitting that He Who commanded us to honor our father and mother should not in His Birth lessen the honor due to His Mother.

http://www.newadvent.org/summa/4028.htm#article2

The Thomistic rationale is pretty ad hoc. Regarding the third rationale, does childbearing dishonor a mother? Didn’t God design this process? Doesn’t this design antedate the fall?

As for the first two reasons, Aquinas seems to be saying that ordinary childbirth would defile both the Christchild and his mother.

But it’s hard to see the logic of that, even on Catholic grounds:

i) A body qua body is not sinful. What we do with our bodies can be sinful, although childbirth is hardly sinful.

ii) Since Christ is both sinless and impeccable, how could passage through the birth canal defile him?

iii) Since, according to the Immaculate Conception, Mary is sinless, how could her birth canal define the Christchild?

The only plausible explanation I can proffer for these inconsistent positions is that each dogma represents a legendary embellishment which evolved independently of the other, without thought for the other. And that’s why they don’t go together.