Then why go through the charade of betrothal and marriage. The command even before sin was that (Gen. 2:24,) "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." One flesh is one flesh.
Do you think God selected Mary to bare Jesus after she was betrothed to Joseph? If before, and it was God's plan all along that she was to remain a virgin, then Joseph was defrauded because he didn't find out she was pregnant until after the betrothal and there is no scripture that says he was told she was eternally a virgin.
Touching the holy makes one holy it does not desecrate the holiness.
Because a teenage mother out of wedlock in Israel 2,000 years ago needed a man to belong to, to protect her, to give her legitimacy; because God knew St. Joseph would accept his chosen role and because he loved God and Mary. Any number of reasons for the "charade."
Touching the holy makes one holy it does not desecrate the holiness
Remind Judas Isacriot.
We know that God's plan in general, and so in application to Virgin Mary was from before the foundation of the world. By your logic then Joseph was "defrauded" whether or not he had carnal relation with Mary after Christ's birth. If you consider step fathering the Savior a tort of fraud is up to you, but your argument still doesn't make any scriptural sense.