Why was it neccessary for Mary to remain a virgin after the birth of Jesus? Is there something inherently less-than-Godly in becoming one flesh with a lawful spouse, i.e. sexuality is detrimental to righteousness?
Its sounds as though you need to ponder the last question in the article some more.
However, to give you a short answer, read the entirety of 1 Corinthians 7. Marriage is "honourable in all" (Hebrews 13.4), but Virginity is "more blessed" (1 Cor. 7.40). Two snippets are below.
1 Now concerning the things whereof you wrote to me: It is good for a man not to touch a woman.
2 But for fear of fornication, let every man have his own wife: and let every woman have her own husband.
3 Let the husband render the debt to his wife: and the wife also in like manner to the husband.
4 The wife hath not power of her own body: but the husband. And in like manner the husband also hath not power of his own body: but the wife.
5 Defraud not one another, except, perhaps, by consent, for a time, that you may give yourselves to prayer: and return together again, lest Satan tempt you for your incontinency.
32 But I would have you to be without solicitude. He that is without a wife is solicitous for the things that belong to the Lord: how he may please God.
33 But he that is with a wife is solicitous for the things of the world: how he may please his wife. And he is divided.
34 And the unmarried woman and the virgin thinketh on the things of the Lord: that she may be holy both in body and in spirit. But she that is married thinketh on the things of the world: how she may please her husband.
35 And this I speak for your profit, not to cast a snare upon you, but for that which is decent and which may give you power to attend upon the Lord, without impediment.
36 But if any man think that he seemeth dishonoured with regard to his virgin, for that she is above the age, and it must so be: let him do what he will. He sinneth not if she marry.
37 For he that hath determined, being steadfast in his heart, having no necessity, but having power of his own will: and hath judged this in his heart, to keep his virgin, doth well.
38 Therefore both he that giveth his virgin in marriage doth well: and he that giveth her not doth better.
39 A woman is bound by the law as long as her husband liveth: but if her husband die, she is at liberty. Let her marry to whom she will: only in the Lord.
40 But more blessed shall she be, if she so remain, according to my counsel. And I think that I also have the spirit of God.
Blessed Mary and St. Joseph did not make use of their marriage because it was more blessed not to, because they could more easily in this manner attune their minds in prayer to their son, God the Son, living under their roof, and because it would have been unfitting to do so once God had made holy the womb of Mary by his miraculous conception within it.
Is sexuality detrimental to righteousness? Yes and no. Yes, in that it tends to take the mind away from God and to carnal pleasure. This necessitates the periodic abstinence St. Paul speaks of in verse 5 to redevote ourselves to prayer. No in that it is the natural and sacramentally blessed manner of man and wife expressing their union which symbolizes the eternal union of Christ with his Church (Ephesians 5.21-33).
A question back to you in return. If St. Joseph and Blessed Mary had intercourse after the birth of Christ, where are his supposed brothers and sisters 13 years later when on the journey back from Jerusalem, the Holy Family is clearly portrayed as St. Joseph, Blessed Mary, and Jesus only (Luke 2.41-52)?
43 And having fulfilled the days, when they returned, the child Jesus remained in Jerusalem. And his parents knew it not.
44 And thinking that he was in the company, they came a day's journey and sought him among their kinsfolks and acquaintance.
45 And not finding him, they returned into Jerusalem, seeking him.
Verse 44 is the perfect place to note Jesus' brothers and sisters, had he had any. None are mentioned. Verse 45 is explicit that the family consisted of Jesus, who was missing, and Mary and Joseph, who went to find him.
Like normal parents, they assumed their young man was among the other relatives on the journey. But when they missed him the very first night, they alone returned immediately to Jerusalem to find him. Their concern for Jesus was enough to prompt this reversal of their return trip. Had they other younger children, it seems inconceivable they would have left them with relatives to continue onwards, when they were not willing to let Jesus be out of their company with relatives more than a days time.
12 years interval from the birth is a LONG time for no natural conceptions to occur, especially in a young woman of Blessed Mary's age. The obvious conclusion is that she and St. Joseph were not having intercourse because at the very least, Blessed Mary had vowed her self to perpetual virginity.
Luke 1.34 And Mary said to the angel: How shall this be done, because I know not man?
Mary is not punished for her question, as Zachariah was in asking about the conception of St. John the Baptist, as related earlier in Luke 1:
18 And Zachary said to the angel: Whereby shall I know this? For I am an old man, and my wife is advanced in years.
19 And the angel answering, said to him: I am Gabriel, who stand before God and am sent to speak to thee and to bring thee these good tidings.
20 And behold, thou shalt be dumb and shalt not be able to speak until the day wherein these things shall come to pass: because thou hast not believed my words, which shall be fulfilled in their time.
What is the difference here? Zachariah doubted the word of the Lord because he was old, forgetting the lesson of Abram and Sarah. Blessed Mary's question would fall into the same category of doubting God if she had any expectation of shortly commencing sexual relations with St. Joseph, to whom she was espoused. Had she that expectation, the question would have made no sense, she would have expected that the conception come about in the normal manner, as Zachariah obviously did (although he doubted his and his wife's capabilities because of age).
Blessed Mary's question, and the Archangel Gabriel's response, only makes sense if a moral impediment existed to Mary ever conceiving a child in the normal manner, despite her upcoming marrage, namely a vow of perpetual virginity on the part of Blessed Mary. Were that the case, the question in Luke 1.34 would then be understood: "How shall I conceive when I have pledged to refrain always from sexual intercourse?"
Now if Blessed Mary had made such a vow, and I believe the evidence points to it, we would know it rested upon the premise that St. Paul explained in 1 Corinthians 7 - that virginity is more blessed than marriage and enables one to more fully to attend to God. Certainly this is why Elijah, Elisha, Jeremiah, and St. John the Baptist all did not marry. Mary's marriage would then be seen as her or her parents seeking a natural protector for her to enable her to live this vow unmolested by the world.
This would also explain the thought of St. Joseph upon learning of her pregnancy as recounted in Matthew 1, probably shortly after the 3rd month, when Blessed Mary returned from being with St. Elizabeth after the Birth of St. John the Baptist:
18 Now the generation of Christ was in this wise. When as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost.
19 Whereupon Joseph her husband, being a just man, and not willing publicly to expose her, was minded to put her away privately.
Why would Joseph be considering putting Mary away once she became pregnant? It only makes sense if he had understood his role as that of marrying an avowed virgin to offer her protection and comfort towards the world's hostility. How bewildering to then find pregnant after a long absence, the one whom he asusmed we never be in such a state! His action only makes sense if he could not believe that given her known character Blessed Mary could commit adultery, which would require a public condemnation, and knew that the pregnancy was not the result of rape; and yet how he must have been puzzled at that pregnancy!
It appears that it was still the night that he discovered her pregnancy that the truth of it was revealed, because the Bible says in Matthew 1:
20 But while he thought on these things, behold the Angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost.
It could not have been very long yet, and he must have decided to sleep on the conundrum, hoping to have a good resolution in the morning. He did. The vow of virginity of Mary had not been broken, nor would it be broken.