So what’s the big issue that Mary had sex with her husband?
Does it make her a better person?
Does it make her more holy?
Would it have been sin for her to do that?
How would it affect Jesus’ redemption of mankind if she had sex after He was born?
Mary is the Ark of the New Covenant. She carried the Second Person of the Holy Trinity for nine months. Her soul and her body were - and remain - holy and united to God in an intimate way.
In the Old Testament the Ark of the Covenant was so holy that men were struck down for simply touching it.
How much more holy, then, is the living ark of the Incarnate Jesus Christ?
Understand that Mary brought Christ to mankind. She was set aside by God almighty.
People (and things) that are devoted to holy purposes are not suitable for natural purposes.
The sacrament of marriage is good and so is the procreation of children. Yet Mary’s condition and office as the Mother of the Son of God set her apart so that she and Joseph would not and did not enjoy the natural good of conjugal relations.
She was set aside for God. Joseph understood that. Other Christians have understood that for two millennia.
Believing that 'Mary and Joseph must have had sex' - with no evidence! - simply reveals a colossal lack of understanding of what the Incarnation is.
So then Joseph was a perpetual virgin too?