Why is it necessary for Mary to be perpetually virgin once Jesus was born?
Why would it be wrong for her to have sex with Joseph?
Because that would be contrary to her being an object of worship, as sex would defile her. Consider the views of some so-called church fathers:
Jerome saw marriage as so inferior (at the least) to virginity, celibacy and continence, that he engaged in specious reasoning and abused Scripture to support his extreme imbalanced views, teaching,
"It is not disparaging wedlock to prefer virginity. No one can make a comparison between two things if one is good and the other evil ." (''Letter'' 22). On First Corinthians 7 he reasons, "It is good, he says, for a man not to touch a woman. If it is good not to touch a woman, it is bad to touch one: for there is no opposite to goodness but badness. But if it be bad and the evil is pardoned, the reason for the concession is to prevent worse evil."
You surely admit that he is no bishop who during his episcopate begets children. The reverse is the case—if he be discovered, he will not be bound by the ordinary obligations of a husband, but will be condemned as an adulterer.
Then we have this false dilemma:
"If we are to pray always, it follows that we must never be in the bondage of wedlock, for as often as I render my wife her due, I cannot pray.
And wresting of Scripture to serve his purpose:
This too we must observe, at least if we would faithfully follow the Hebrew, that while Scripture on the first, third, fourth, fifth, and sixth days relates that, having finished the works of each, “God saw that it was good,” on the second day it omitted this altogether, leaving us to understand that two is not a good number because it destroys unity, and prefigures the marriage compact. Hence it was that all the animals which Noah took into the ark by pairs were unclean. Odd numbers denote cleanness. St. Jerome, Against Jovinianus Book 1 http://www.ccel.org/ccel/schaff/npnf206.vi.vi.I.html
So much for 2 x 2 evangelism, while "if we would faithfully follow the Hebrew" "God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day." (Genesis 1:31)
Then we have Augustine who taught that one cannot engage in marital relations without sinful lust:
the very embrace which is lawful and honourable cannot be effected without the ardour of lust, so as to be able to accomplish that which appertains to the use of reason and not of lust....This is the carnal concupiscence, which, while it is no longer accounted sin in the regenerate, yet in no case happens to nature except from sin. — On Marriage and Concupiscence (Book I, cp. 27); http://www.newadvent.org/fathers/15071.htm
For the belief was, as Harding (below) holds, "before they sinned, Adam and Eve had perfect command of their passions (reproductive actions]." But having lost that due to the Fall, then men as Augustine held that martial relations must involve carnal sinful lust, and even interprets Heb. 13:4 which states that the marriage bed is undefiled (unlike under the Law) to simply mean if it is free from adultery!
However, if martial relations was painful I do not think he would considered it iniquitous. But as per the logic that a function which at the last is uncontrollable is sinful, perhaps another daily bodily function of relief which can uncontrollable (if you cannot find a bathroom) is also sin.
Similarly, Tertullian argued that second marriage, having been freed from the first by death, "will have to be termed no other than a species of fornication," partly based on the reasoning that such involves desiring to marry a women out of sexual ardor. An Exhortation to Chastity,'' Chapter IX.—Second Marriage a Species of Adultery, Marriage Itself Impugned, as Akin to Adultery, ANF, v. 4, p. 84.]
. Also regarding some strange views on the issue of Adam and Eve and sexual relations, RC priest John A. Hardon, S.J stated, "some of the Fathers [as Athanasius and John Damascene] were so firmly persuaded of the natural integrity of our first parents that they derived marriage from original sin." (Harding: http://www.therealpresence.org/archives/God/God_013.htm)
For John of Damascus wrote,In Paradise virginity held sway. Indeed, Divine Scripture tells that both Adam and Eve were naked and were not ashamed416 . But after their transgression they knew that they were naked, and in their shame they sewed aprons for themselves417 . And when, after the transgression, Adam heard, dust thou art and unto dust shalt thou return418 , when death entered into the world by reason of the transgression, then Adam knew Eve his wife, and she conceived and bare seed419 . So that to prevent the wearing out and destruction of the race by death, marriage was devised that the race of men may be preserved through the procreation of children420.
...God, Who knoweth all things before they have existence, knowing in His foreknowledge that they would fall into transgression in the future and be condemned to death, anticipated this and made “male and female,” and bade them “be fruitful and multiply.” — John of Damascus, Exact Exposition of the Orthodox Faith, Book IV, Chapter XXIV; http://www.trueorthodoxy.info/cat_stjohndamascus_exact_exposition_Orthodox_Faith_bk04.shtml
Many Caths make Mary into being the fulfillment of the the Ark of the New Covenant since the Lord dwelt therein (which could thus be said of the church, the temple of the Holy Ghost), which they crown with gold, and which was not to be touched at pain of death (though actually qualified men did without dying), meaning Joe maybe died because he tried to get intimate with his wife (yet Scripture also says, "Touch not my anointed" = church). But the Ark best represents Christ, gold representing His glory, as Christ is the brightness of God's glory, and the express image of His person, (Heb. 1:3) and who contained the law and the words of life, and the rod of God as did the Ark. (Heb. 9:4) And by whom God spoke to man, as the word was made flesh, taking on the common “wooden” body of man.