Old Yeller - that is not what the Church teaches.
The explanation is drawn from both the Bible and early Christian testimony.
In Matthew 1:18 we read: "When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child of the Holy Spirit."
So Joseph and Mary definitely didn't consummate their marriage prior to Jesus' birth. What about after?
you look at Matthew 1:25 as a suggestion that Joseph and Mary "knew each other" after the birth of Jesus: "but knew her not until she had borne a son; and he called his name Jesus."
However, This means only that Joseph had nothing to do with the conception of Jesus. It implies nothing as to what happened afterward.
The expression "until" is used a lot in the Bible. For example, in Genesis 26:12-13 we read how Isaac "sowed in that land, and reaped in the same year a hundredfold. The Lord blessed him, and the man (Ambimelech) became rich, and gained more and more until he became very wealthy." Does this "until" imply that Ambimelech ceased to prosper? Likewise, the "until she had borne a son" does not necessarily mean Joseph and Mary had relations after the birth of Jesus.
St. Jerome (d. A.D. 420) made this point about 1,600 years ago: "In like manner ‘until’ often denotes in Scripture...a fixed period, but often also an infinite time, as in that, Even to your old age I am He. (Isa. 46:4.) Will God then cease to be when they are grown old? Also the Savior in the Gospel, Lo, I am with you always, even to the end of this world. (Mat. 28:20.) Will He then leave His disciples at the end of the world?" (St. Jerome, quoted by St. Thomas Aquinas in the Catena Aurea.)
Anysis the Bishop of Thessalonica wrote in A.D. 392 that Mary conceived no other children after the birth of Jesus Christ. This was proclaimed by the whole Church in the Second Council of Constantinople of A.D. 553, and reaffirmed in the Lateran Council of A.D. 649. (The Sources of Catholic Dogma, Roy Deferrari, Trans., from Enchiridion Symbolorum, Henry Denziger, Ed. (Fitzwilliam: Loreto Publications), 39, 87, 101.)
Both Joseph and Mary consummate their marriage by fulfilling the special mission of taking care of Jesus Christ. It was the mission assigned to them by God.
Joseph had a lear knowledge about who Mary is. He knew her as the mother of Jesus Christ consumed by the Holy Spirit. Joseph had more respect to Mary, the Mother of God
The Catholic Church DOES not teach that sex between a husband and wife is dirty. The commune of Ole Yeller (membership 1) may think so, but the Church does not
teaching them to observe all that I have commanded you.[a] And behold, I am with you always, until the end of the age.”
So after that The Lord wont be with us? Of course not.
What a sad, pathetic life he must lead--to feel the need to excoriate fellow Christians. Thank goodness our Lord has much mercy...and justice.