I hope you don't mind an answer from a Catholic, seeing we agree on this with the Orthodox Christians.
To ask why something needs to be the case, or the truth, is strange. But I see you are asking why it is so integral that Mary be believed to be ever-virgin, or, perpetual virgin, not why it is true.
The answer is because it is true. That is, because it's true it's not negligable like the rest of the Christological or Mariological facts (they are more related than you'd think). It's believed and recieved just as much as the written Word of God was recieved.
Among the Fathers of the Church it was simply the orthodox (correct, traditional) consensus and a part of the Faith. See for example St. Jerome's Against Helvidius, for a more formal rebuttal of the inverse of the orthodox truth; the heresy of denying the perpetual virginity of Mary.
(I personally argue that the fact she was consistently called "the Virgin" and variations of that 'title' more than strongly imply this truth that she did not have other children apart from Christ.)
"If church tradition held that Jesus had blue eyes, I would have no
reason to doubt it, but I wouldn't insert "oh blue eyed one" into my
prayers to Jesus."
But imagine if the someone claimed the Ark of the Covenant was made of perishable wood and was covred in clay instead of gold. And imagine if the Ark was a person.
Well, Mary is shown in the New Testament to be, undeniably, the Ark of the New Covenant. With implications.
For preliminary reasons:
ARK OF THE [OLD] COVENANT
● A symbol of the Presence of God making His dwelling with man
(NEW) ARK OF THE NEW COVENANT (MARY)
● In which God was truly and physically present with us
● God’s Glory Cloud overshadowed (Greek Septuagint episkiasei) the Tent and the Glory of the Lord filled the Tabernacle (abode) of God with the Ark of the Covenant in it
● The Spirit of God overshadowed (Greek NT episkiasei) Mary and she was filled with, and was made the dwelling place of “the Lord of Glory” in her home.
● Ark was being brought to the house of Obed-edom in the hill country of Judea
● Mary travelled to the house of Elizabeth and Zechariah in the hill country of Judea
● Cymbals and harps and lyres etc were played to play music before the Ark, and right after:
● David danced and lept joyously before the Ark
● At the sound of Mary’s voice:
● John the Baptist lept for joy in Elizabeth’s womb in Mary’s presence
● David rejoices with “shouts and sound of the trumpets” in the presence of the Ark
● Elizabeth “exclaimed with a loud voice” to Mary praises of her being “blessed among women” (cf. Judith 13:18)
● Stays at the house of Obed-edom for three months
● Stays at the house of Elizabeth and Zechariah for three months
● Contained the Word of God (Tablets of the Old Covenant and Law) engraved on stone
● Contained the Word of God made flesh, who brought the New Law of His New Covenant
● Contained the rod of the highpriest Aaron
● Contained the "great Highpriest...Jesus the Son of God"
● Contained Manna (their sustenance/food) from Heaven
● Contained the Bread of Life, Jesus, “who [came] from heaven”, who was born in BethLechem ('house-of-bread').
● God’s presence literally dwelt on the Ark
● God literally dwelt inside Mary
● Is covered within and without with the purest gold by virtue of its being the dwelling/visiting place of the Most High God.
● Of most pure virtue, and (perpetual virginity (Ezekiel 44:1-2), and without stain sin by virtue of the One to whom she would give flesh and be the mother of.
Or said beautifully by Athanatius of Alexandria: “O noble Virgin, truly you are greater than any other greatness. For who is your equal in greatness, O dwelling place of God the Word? To whom among all creatures shall I compare you, O Virgin? You are greater than them all O [Ark of the] Covenant, clothed with purity instead of gold! You are the Ark in which is found the golden vessel containing the true manna, that is, the flesh in which Divinity resides.” (Athanasius of Alexandria, Homily of the Papyrus of Turin ~370AD)
Other very significant prophetical elements of the Ark of the Covenant:
ARK OF THE [OLD] COVENANT
● Exodus 25:10 "And they shall make an ark of shittim wood..." Even in the Septuagint, this is rendered 'incorruptable wood', i.e. imperishable wood, because it is a specific type of wood that doesn't rot. (Exodus 37:1)
● Exodus 37:2 And he overlaid it with pure gold within and without, and made a crown of gold for it round about."
● The people prayed as though through the medium of the Ark to the Lord (Joshua 7:6-9, 1 Chronicles 13:3, Judges 20:27) and it was the cause of many miracles (Dividing the Jordan Joshua 4:7, Fall of Dagon 1 Samuel 5:1-4, Bringing Down of the Walls of Jericho Joshua 6:6-20 etc)
ARK OF THE NEW COVENANT (MARY)
● All existing Traditions about Mary are such that Mary did not decay/see corruption (but she did die, like her Son did) because she did not suffer this consequence of Original Sin, from which she was preserved.
● Mary is the Queen in the Davidic Kingdom, since Christ now sits as King, and in the Davidic Kingdom, the mother of the king was the Queen. Mary is the new Queen (Psalm 45:9, Revelation 12:1, ) and mother of those in Christ, who is our best advocate with Christ, as His holy and beloved mother (1 Kings 2:17–20).
● People have always prayed to Mary as a chief intercessor on their behalf with Christ ("Pray for us that we may be worthy of the promises of Christ", "Pray for us sinners now and at the our of hour death" etc) as a most potent means of asking God with great success, even when we ourselves are far less worthy to be receive than His blessed mother to ask. She has worked many miracles throughout the Christian era, the era of Christ.
The fulfillment of an Old Testament type is always, without exception greater in every sense to its former shadow, "even as one's shadow is inferior to himself", as Thomas Aquinas said.
So if the Ark was the "glory" of Israel, and was "the holy Ark" (2 Chroincles 35:3), then Mary was also extremely holy indeed. (same "you are the glory of Israel" is said of Judith (Judith 15:9), an amazing type of the Blessed Virgin Mary, too!)
Now given this, might we expect the Ark to be somehow connected with Mary, the bearer of the Christ and His mother, to be made somewhere explicitly? Well, we find such a thing in Revelation 12! Consider a little context, firstly:
In chapter 11, which precedes it, the context is unrelated to the Ark, and relates the unleashing blowing of the angel's trumpet,woes etc.
Notice, it is a short chapter, and ends abruptly with:
"Then there was opened the temple of God that is in heaven; and there was seen in his temple the Ark of his covenant; and there followed lightnings, and voices, and thunders, and an earthquake, and great hail."
The end. Now, I don't know about you, but that is an abrupt and unnaturally mysterious ending to a chapter(even for Revelation). Well, we're in luck. Luckily, the Apostles didn't separate their books of the New Testament into chapters or verses.The original manuscripts are just free-running texts. In other words, the 'chapters' and 'verses' are for reading ease _and are man made—centuries later additions for translations.
Well, it seems John did not end there when he introduced the Ark.
He 'explains' it with a certain 'Woman'.
So just after revelation 11:19, the original manuscripts does not change to a different chapter, it continues (to Revelation 12:1...)!
"And a great sign appeared in heaven: A woman clothed with the sun*, and the moon under her feet, and on her head a crown of twelve stars: she was pregnant and crying out, and in pain to give birth." (v1)
This particular Woman gives birth to Jesus Christ. This Woman is Mary. This Woman has overtones of the church, too, and has been interpreted as both at the same time (we're dealing with highly symbolic stuff here in revelation, after all):
"And she [the Woman] brought forth a male child, who was to 'rule all nations with an iron rod': and her son was taken up to God, and to his throne." (v5)
Clearly, her Son is Jesus Christ. Clearly, this is symbolizing Mary, then. Funny how she is the description of the vision of abrupt vision the Ark as we might predict!
* Symbolic of her being the ark which was covered with pure gold? (cf. Ps 45:9; Ex 25:11)
Why am I talking about the Ark of the Covenant?
Well, people who suppose that this pure Ark, Mary, could lie intimately with a man, is to ignore the great reverence and veneration due to the Ark—people were stricken dead by God on the spot for even peering into or touching the Ark, even for 'legitimate' or 'licit' reasons. It is where God Almighty dwelt. Moses had to take his sandals off for even walking on the ground God was present at.
Speaking of legitimate reasons: Is, as you touch on, virginity/'celibacy' related to a higher spiritual calling or is is more valuable or precious. And the answer is yes, it is a higher calling. St. Paul argues exactly so in 1 Corinthians 7:6-8. And so does Jesus in Matthew 19:~10-12. Virginity is also equated with holiness in Revelation 14:4 and elsewhere.
Finally, Mary more than implies she intends to remain virgin, or had taken some kind of vow of virginity. We read when the angel Gabriel greeted her and told her she would concieved and bear a Son, she asked:
"How can this be, seeing I know not man?" (Luke 1:34) This is a Hebraism or euphemism for "I do not have/have not had sexual relations". And she asks it even though already betrothed/married to Joseph (and thus she would know this child would be the result of the consummation of that marriage). The only way to make sense of her question is to acknowledge she had taken a vow of virginity.