1 Corinthians 7:5
Defraud ye not one the other, except it be with consent
for a time,
that ye may give yourselves to fasting and prayer;
and come together again,
that Satan tempt you not for your incontinency.
James 4:17 Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
And Mary was not the Ark of the New Covenant. Jesus is the mediator of the New Covenant.
"Hail Mary, full of grace!"
That is the angel's greeting to her. Through the angel, God asked Mary to bear His Son, the Salvation of mankind. And what was her response? "Be it done unto me according to thy word". Mary is the ark by virtue of the fact that she carried God in her womb for 9 months and then nurtured Him at her breast. Do you for one second believe that God chose to be born from some woman randomly selected? He is the Creator of the Universe!
Jesus' unique Sonship from Mary reflects His unique Sonship in eternity. Christ is the only-begotten Son of the Father, who begets Him eternally without the help of a mother. He is also the only Son of Mary, who conceives Him in time without the help of a man.
It is also further affirmation of the holiness and Deity of Jesus. It would not have been fitting for the womb which bore the Savior to bring sinners into the world (which any hypothetical child of Joseph and Mary would have been). As the ancient ark of the covenant was consecrated for sacred use, so the New Ark could not be defiled by common usage.
You are quite skilful at selectively snipping bits of scripture out of context. Read the whole chapter.
First off, you avoid quoting the very next verse where Paul says “But I say this as a concession, not as a commandment.” He nowhere states it is a sin to not come together. That is your assumption.
Next, the limit “for a time” is not defined. Is it weeks, months, years? We know that Mary and Joseph did not come together for at least 9-months because of the virgin birth. Did that break Paul’s “concession”? I doubt it, because the key phrase of the verse is come together again, that Satan tempt you not for your incontinency (or self-control per other translations). We know that Mary and Joseph stayed apart for at least 9-months, so we know they had self-control. It is quite possible they maintained self-control after His birth. Scripture doesn’t say one way or the other.
In any case to say Mary and Joseph would sin by violating Paul’s concession is absolutely wrong and figment of your imagination.