Now I'm surprised. You had a virginal pregnancy in your family?
"Do Catholics have some sort of tradition or document about the marriage agreement between two specific people two thousand years ago?"
Not at all. But *you're* the one who said that if Mary did not have sexual intercourse with Joseph, she would "breach the marriage contract that both had consented to at the beginning. Unless Mary wanted to breach said contract but that would be a sin." You imply you know what was in Mary and Joseph's contract. Did you see it? I doubt it.
"...your words sound pretty crass about "the Ark delivered the goods."...
You made the Ark analogy, not me." It's not the Ark analogy that's crass, it's the idea, stated by you, that after she was of no use as "Ark," she had no special reason to remain faithful to her Bridal relationship with the Holy Spirit, which we know from Scripture to be a lasting one (Rev. 22:17)- "The Spirit and the Bride say, come." In this passage, the image is both of the Spirit's actual Bride, Mary, the one Our Lord the Holy Spirit overshadowed and impregnated; and also of the Church, because in so many, many instances Mary is the type and image of the Church.
Mary/Church and Church/Mary appears over and over again in the overlapping language of Biblical symbolism. They are the personal and the corporate image of those united to God by faith.
So the "crass" thing is not the analogy between Mary and the Ark of the Covenant. The crass thing is suggesting that once Mary gave birth, she had "delivered the Goods' and was no longer Daughter of the Covenant, Bride of the Spirit, Mother of the Messiah King. (All of which is Scriptural.) In this crass supposition, she could be tossed back like a "surrogate" who completed her contract. As if God would say --- and this is blasphemous --- "I got in there first and got what I wanted, Joseph, now you can have her.") Caling Mary a "demi-Goddess" shows an ignorance of the glorious nature of the "Great Things" which "God who is Mighty" had done for her. Mary is the humble one. Mary is a handmaid. Mary's spirit exults in God her Savior. This is the most beautiful glimpse we get, of how God exalts the humble.