This is a unique situation and has nothing to do with the sanctity of Marriage. The virginity of Mary not only expresses the singular and sublime work of God in the incarnation (and the vessel He worked through), but also expresses something about the Church itself, since Mary and the Church are interwoven throughout Scriptures. As I have said here before, what the Church says about itself can also, in a spiritual sense, be said about Mary. Thus, the idea that Mary REMAINED totally committed to God says something about the Bride of Christ.
Regards
Nevermind that the concept is unbiblical.
Catholic Doctrine: Mary never sinned.
Biblical Doctrine: Romans 3
10As it is written, There is none righteous, no, not one:
11There is none that understandeth, there is none that seeketh after God.
12They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.
13Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips:
14Whose mouth is full of cursing and bitterness:
15Their feet are swift to shed blood:
16Destruction and misery are in their ways:
17And the way of peace have they not known:
18There is no fear of God before their eyes.
19Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
20Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
21But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
22Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:
23For all have sinned, and come short of the glory of God;
Biblical Statement from Mary:
Luke 1:47 And my spirit hath rejoiced in God my Saviour.
Catholic Doctrine: Mary was a perpetual virgin and had no other children
Biblical Statement:
Matthew 1
24Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife:
25And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
Luke 2:7 and she gave birth to her firstborn, a son.
Furthermore, Cross-referencing Luke 2 with Leviticus 12 shows that Mary was considered unclean after the birth of her son and brought a sacrifice to the temple in order to atone for herself. If she were sinless, this would have not only been a fruitless exercise but would have been blood unnecessarily spilt.