When did Mary and Joseph learn that they were never supposed to have marital relations, according to Roman Catholic doctrine?

Church has witnessed to the perpetual virginity of the Theotokos steadfastly for two thousand years and shows no sign of tiring. In the West, the idea was largely undisputed until late in the Reformation; even Luther and Calvin accepted the tradition.

Indeed, to suggest (a) that the tradition about her perpetual virginity could have been introduced after apostolic times, (b) that this tradition would have gone little noticed by a Church in the throes of questioning everything about what it believed in the first millennium, (c) that such a novel tradition should be considered inconsequential enough to pass without discussion before it became universally proclaimed, and (d) that such a tradition should have no discernible literary or geographical origin and yet be universally accepted from very early in the Church's history, is to form a very unlikely hypothesis.

Set Apart to God

To argue against Mary's perpetual virginity is to suggest something else that is greatly implausible, not to say unthinkable: that neither Mary nor her protector, Joseph, would have deemed it inappropriate to have sexual relations after the birth of God in the flesh. Leaving aside for a moment the complete uniqueness of the Incarnation of the Second Person of the Trinity, recall that it was the practice for devout Jews in the ancient world to refrain from sexual activity following any great manifestation of the Holy Spirit.

Source: goarcch.org

The book of Matthew 1:24-25, says

"24 Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: 25 And knew her not till she had brought forth her first born son: and he called his name JESUS."

The above passage told us the extent to which Joseph deprived himself of intimacy (verse 25) and there was nowhere in the Bible where Joseph was told to never touch her neither did the Bible say he didn't!
Rather Matthew 13:54-56 clearly mentioned Jesus' mother Mary, his father's profession (carpenter) and his brothers James, Joses, Simon, and Judas. Sister's were not mentioned naturally as the Jews give less attention to that. These were immediate family blood relations and not extended families.

If God had no desire for Joseph to be intimate with Mary, there would have been no need for marriage. The main reason for the whole 'drama' haven fulfilled, Joseph and Mary lived their normal lives.

I believe the Bible was silent about Jesus' family because that was not the focus of the scriptures and this is proven in Mark 15:40 where Mary and her children were mentioned in relation to Matthew 13:55.

According to Catholic teaching, openness to having children is required for a valid marriage:

1917 Can. 1081 § 2
Matrimonial consent is an act of the will by which each party gives and accepts perpetual and exclusive rights to the body, for those acts that are of themselves suitable for the generation of children.

Explaining how Sts. Mary and Joseph had a valid marriage, despite intending to remain virgins, St. Thomas Aquinas writes (Super Sent., lib. 4 d. 30 q. 2 a. 1 qc. 2 ad 2):

Before the Blessed Virgin contracted [marriage] with Joseph, she was divinely certain that Joseph held the same intention [to preserve virginity]; and thus no danger is committed marrying. Nor because of this is any truth diminished, because that proposition was not conditionally applied in the consent*; when such a condition be against a marriage good (e.g., procreating children), it would invalidate the marriage.

^{beata virgo antequam contraheret cum Joseph, fuit certificata divinitus quod Joseph in simili proposito erat; et ideo non se commisit periculo nubens. Nec tamen propter hoc aliquid veritati deperiit, quia illud propositum non fuit conditionaliter in consensu appositum: talis enim conditio cum sit contra matrimonii bonum, scilicet prolem procreandam, matrimonium tolleret.}

^{*i.e., they didn't say: "I will marry you provided we both remain virgins."}

^{cf. Ford S.J.'s Validity of Virginal Marriage}

Mary, by her own free will, took a vow of virginity, and she understood that this meant that she was never supposed to have marital relations. Joseph knew when he was informed of Mary's vow. Both came to understand that marriage was their vocation, and became betrothed.

This knowledge has come down through tradition and Scripture. The following point is worth making:

As to Mary, St. Luke (1:34) tells us that she answered the angel announcing the birth of Jesus Christ: "how shall this be done, because I know not man". These words can hardly be understood, unless we assume that Mary had made a vow of virginity; for, when she spoke them, she was betrothed to St. Joseph. _{(Catholic Encyclopedia)}

Such a vow would have served the religious purpose of bringing the one making it closer to God. Indeed, of all people, Mary was and is closest to Jesus. She was first to know of His Incarnation, He was born of her, raised by her; her soul was pierced (Luke 2:34-35) with grief at His passion and death.

One might ask why Mary consented to her betrothal, though she was bound by her vow of virginity. As she had obeyed God's inspiration in making her vow, so she obeyed God's inspiration in becoming the affianced bride of Joseph. Besides, it would have been singular among the Jews to refuse betrothal or marriage; for all the Jewish maidens aspired after marriage as the accomplishment of a natural duty. Mary trusted the Divine guidance implicitly, and thus was certain that her vow would be kept even in her married state. _(ibid)

Pope John Paul II writes that Mary and Joseph would have understood Mary's intention at the time they were betrothed. On the difficulty of understanding Mary's inner motivations for accepting, he writes:

We can wonder why she would accept betrothal, since she had the intention of remaining a virgin forever. Luke is aware of this difficulty, but merely notes the situation without offering any explanation. The fact that the Evangelist, while stressing Mary's intention of virginity, also presents her as Joseph's spouse, is a sign of the historical reliability of the two pieces of information. _(source)

Some seem to object to the idea that two persons could be married if marital relations were, by the consent and duty of both persons, out of the question. I have been unable to find any basis in tradition or Scripture for this view that such a marriage would be forbidden or even undesirable. God chose a couple in a certain especially pious and provably obedient state of marriage. This makes perfect sense to me and to Catholics in general.

As for the other part of the question, it invites those answering to speculate on temptations and desires, which by their very nature are private. Their marriage was pure, Mary was without sin, and Joseph was her most chaste spouse. What more can be said, and what more needs to be said?

Mary and Joseph were brought together by God's will, and became the mother and the foster-father of Jesus Christ. Both were aware of Mary's vow of virginity, and of what that vow meant for their marriage.