I think the foremost reason that the blessed mother’s perpetual virginity is important is because it puts into question the very veracity of Christ as the messiah prophesied.
We see from Scriptures that it is likely that the blessed mother had made a vow of virginity even before the angel of the Lord visited her which is evidenced by her answer to the angel saying, “how can this be done because I know not man?” St Augustine and subsequent fathers of the Church and theologians have reasoned from this response that she indeed had made a vow of virginity and subsequently it would be unreasonable for one so faithful and obedient to God to have at any time abdicated her vow.
According to St. Thomas Aquinas’ Summa Theologiae there are three reasons why she was forever virgin:
First, it would follow that since Jesus was the only begotten Son of the Father that He should also be the only begotten Son of His mother.
Secondly, the virginal womb of the blessed mother is the shrine of the Holy Spirit where Christ was formed and subsequently nurtured.
Thirdly, such a premise would be derogatory to the dignity and holiness of God’s mother. It would suggest that she was ungrateful and unsatisfied to God for the gift of such a Son.
It is further fitting and logical that the blessed mother of God is the tabernacle of the Lord and her womb is the ark of the New Covenant. Just like the ark carried by the Israelites that contained the law of God, Aaron’s rod and manna from God it should be understood that no man could ever touch what God had consecrated to himself and live. So, it would follow that if St. Joseph, as some have claimed through eisegesis of Scripture, had consummated his marriage to the blessed mother of God and lived then it would put into question even the possibility that Jesus was the messiah prophesied which would mean that the world still awaits the messiah or Savior of the world.
One can find in the writings of Origen and St. Ambrose further explanation of the impossibility that the blessed mother of God was not the perpetual virgin as the Church has always claimed her to be for 2000 years. God bless!