Why is the Catholic teaching that Mary's hymen remained intact during childbirth important? I am asking this question because the entire comment thread in which I asked the question appears to have gone missing, including references to Aquinas (Summa Theologica q. 35 a. 6), Ludwig Ott (Fundamentals of Catholic Dogma bk. 3, pt. 3, ch. 2, §5, 2.), Pohle (Mariology pt. 2, ch. 1, §3, Theses II), and others.
Wikipedia lists St. Lucia of Syracuse (283-304) as the patron saint of of the blind within Roman Catholicism.  She is venerated, along with St. Agnes (patron saint of virgins) among Roman Catholics, Anglican, Lutheran, and Eastern Orthodox churches.  She is one of only 8 women explicitly commemorated by Roman Catholics in the Canon of the Mass.
There is, within the tradition regarding St. Lucia, the possibility that she was assigned to defilement within a brothel by the Governor of Syracuse. Paschasius ordered her to burn a sacrifice to the emperor's image. When she refused, Paschasius sentenced her to be defiled in a brothel; a particularly heinous crime against someone who had dedicated her chastity to God.
In a question regarding the Catholic tradition that Mary (Jesus' mother) did not suffer pain in childbirth (Where does the Catholic tradition that Mary did not have pain giving birth to Jesus come from?), included in the comments of a particular answer, came the assertion that, even if Lucia was raped and even if she had survived and produced a child from this violation, she would still be honored by name in the Catholic Mass as a martyred virgin even though her bodily integrity was ruined.  This was explained as because an intact hymen is accidental to virginity while the commitment of the will is essential to virginity.  In other words the taking of sexual liberty by force and against one's will does nothing to impinge upon one's state of virginity even though it may change the state of one's bodily integrity.  Therefore the state of one's bodily integrity has nothing to say, directly, to one's virginal condition.
The reference to St. Lucia came about as the bodily integrity of Mary (i.e. no ruptured hymen in childbirth) was indicated as integral to her &quot;perpetual virginity&quot; which is in turn linked to her sinlessness which is in turn linked to her painless childbirth.  It seems to me, however, that if an intact hymen is accidental to virginity then a ruptured hymen must surely be accidental to the birth of a virginally conceived child.
If St. Lucia would still retain her virginal status in the eyes of the Catholic Church regardless of the state of her bodily integrity following rape, why is it so important for Mary's bodily integrity to remain intact as regards her &quot;perpetual&quot; virginity during childbirth?
The answer is simply at that time that was the sole measure of virginity, regardless of how some came to think about it centuries later.  Basically, it is important because it was the proof of virginity.
This is shown in scripture and in a very early tradition.

And give occasions of speech against her, and bring up an evil name upon her, and say, I took this woman, and when I came to her, I found her not a maid:  Then shall the father of the damsel, and her mother, take and bring forth the tokens of the damsel's virginity unto the elders of the city in the gate:  Deut 22:14-15

The tokens of virginity included blood and breakage.
We find this same integrity idea in the Infancy Gospel of James.

[19]Then said Salome: As the Lord my God liveth, unless I thrust in my finger, and search the parts, I will not believe that a virgin has brought forth.
20. And the midwife went in, and said to Mary: Show thyself; for no small controversy has arisen about thee. And Salome put in her finger, and cried out, and said: Woe is me for mine iniquity and mine unbelief, because I have tempted the living God; and, behold, my hand is dropping off as if burned with fire.
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This absolutely necessary physical integrity found its way into the Catholic Dogma (de fide).


This union of the mother with the Son in the work of salvation is made manifest from the time of Christ's virginal conception up to his death; first when Mary, arising in haste to go to visit Elizabeth, is greeted by her as blessed because of her belief in the promise of salvation and the precursor leaped with joy in the womb of his mother (cf. Lk. 1:41-45); then also at the birth of Our Lord, who did not diminish his mother's virginal integrity but sanctified it,
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Conceived without blood or breakage.  Birthed without blood or breakage.  Mary's physical virginity remained intact as proof she was and remained a virgin they believe.