So, what does this have to do with Mary’s Perpetual Virginity? Just as the Ark was a holy vessel which bore the Divine Presence, Mary was the holy living vessel chosen to bear the Lord God. The Ark was a created thing, just as Mary was a created being, for the purpose of carrying the presence of God. This explains also the doctrine of the Immaculate Conception of Mary, because only something “perfectly endowed with grace” (Greek kecharitomene, Luke 1:28) was fit to carry God, as the Ark was made only of the purest gold for God’s word (Ex 25:11-21).
Now, if God’s presence “overshadowed” Mary, why did Joseph refrain from intercourse at all? The answer is that even he recognized her special consecration as God’s living vessel, the Ark of the New Covenant. Note that because God’s presence overshadowed the tabernacle, Moses was not able to enter: “Then a cloud covered the tent of the congregation, and the glory of the Lord filled the tabernacle. And Moses was not able to enter into the tent of the congregation, because the cloud abode therein, and the glory of the the Lord filled the tabernacle.” (Ex 40:34-35) Just as Moses was not able to enter the tabernacle due to God’s presence, Joseph was not able to have marital relations with Mary because God’s presence, in the same way as in Exodus 40:34-35, had overshadowed her. Another example is seen in Uz’zah, who aroused the anger of God simply for touching the Ark: “And when they came to Na’chon’s threshing floor, Uzzah put forth his hand to the Ark of God, and took hold of it; for the oxen shook it. And the anger of the Lord was kindled against Uz’zah, and God smote him there for his error; and there he died by the Ark of God.” (2 Sam 6:6-7) Don’t think that Joseph wasn’t aware of this story - the fact that Uz’zah was killed immediately simply for reaching out to steady the Ark being shaken by the oxen. He knew that Mary, as the New Ark of the Covenant, was not to be touched, either. This is why Joseph refrained from intercourse with Mary during her miraculous pregancy and afterwards as well, because if God’s Presence had indeed “overshadowed” Mary’s womb, Catholics believe that Mary’s womb had become the dwelling place of the Lord, something like the Eastern Gate mentioned in Ezekiel 44: “This gate shall remain shut: it shall not be opened, and no one shall enter it; for the Lord, the God of Israel has entered it; THEREFORE, it shall remain shut. Only the prince may sit down in it to eat his meal in the presence of the Lord.” (Ez 44:1-3)