We don't know, but one probability is that it happened in her conversations with St. Luke.
why Mary's normal physical relationship with her legal husband after the birth of our Lord is viewed in any way as wrong or dirty or sinful
Is it really the first time you ask me this? I handled this question half a dozen times on this thread alone. It would not be in the least sinful for Mary to have marital relations with St. Joseph. She remained virgin because she chose to; there was no obligation on her to do so.
And yet it's not recorded in infallible Scripture but what IS recorded in infallible Scripture is this....
Matthew 1:24-2524When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, 25but knew her not until she had given birth to a son. And he called his name Jesus.
and
Luke 2:1-7 1In those days a decree went out from Caesar Augustus that all the world should be registered. 2This was the first registration when Quirinius was governor of Syria. 3And all went to be registered, each to his own town. 4And Joseph also went up from Galilee, from the town of Nazareth, to Judea, to the city of David, which is called Bethlehem, because he was of the house and lineage of David, 5to be registered with Mary, his betrothed, who was with child. 6And while they were there, the time came for her to give birth. 7And she gave birth to her firstborn son and wrapped him in swaddling cloths and laid him in a manger, because there was no place for them in the inn.
And Luke, being the doctor, no doubt knew what the term *firstborn* meant since he didn't say she gave birth to her ONLY son.
How does the church know she stayed a virgin?
The answer is they do not.. This "tradition" is passed down from people that believed sex would somehow soil her.. make her human.
What I think is most noteworthy of the claim by the Roman church that Mary was a perpetual virgin is that it just does not matter..
The state of Mary's sex life has nothing to do with our salvation . She is not our savior, her sins have no impact on our salvation . She was a real flesh and blood human woman, there is not one thing in the plan of salvation that demanded she remain a virgin ..