Thanks, because unless you can provide historical evidence, or Jewish commentary, it really doesn't hold water.
Marriage and Family. Jewish marriages according to rabbinic sources show many formal similarities t the Greek and Roman practices. The marriage was a contract between families. It was effected in two stages: the betrothal (or "acquisition" of the bride) and the wedding proper (taking the bride into the husband's home). The betrothal had the legal force of marriage and could be broken only by divorce (cf. Matt. 1:18-19). It was accomplished by the bridegroom paying the bride-price (or part of it) or delivering a deed. The customary written contract (ketubah) included the husband's duties to his wife and the sum due her in the event of a divorce or his death. The bridegroom moving into his father-in-law's house to live with the bride without further ceremony was also recognized but discouraged in rabbinic writings. The wedding ceremony was held under a canopy (huppah). The bride was prepared by bathing, anointing, and clothing with special adornments. She was then escorted from her father's house to the accompaniment of song, dance, musical instruments, and (since usually in the evening) torchlight. The essential element of the wedding was the introduction of the bride into the groom's house, where the the huppah was set up. Seven blessings were pronounced on the couple and the marriage contract was read, followed by seven days of festivity.
I was probably thinking of the fact the betrothal, unlike our engagement, was a profound legal and financial committment, that required a divorce; but there is no warrant to say that the Virgin Mary shared St. Joseph's house.
Here is a possible reason why the Annunciation had to be close in time to the expected wedding:
I take it as axiomatic that the Holy Spirit so arranged the Incarnation that the virginal conception of the Christ would raise no doubts about Josephs paternity and Marys absolute fidelity to him in the minds of their families and neighbors. This is corroborated by the fact that even three decades later these same people were still unable to accept Jesus as being anyone but the son of the carpenter (cf. Matt. 13:55).There are therefore good grounds for holding that the Spirit must have so arranged the course of events that the angel Gabriel appeared to Mary no more than a few days before the date fixed for the completion of the nuptials, when Joseph would come in procession to her parental home and take her to his house, as this would have allowed virtually the full term to the Childs birth to run its course after Mary had come to live under Josephs roof.
I also found this, by Pope John Paul II
We can wonder why she would accept betrothal, since she had the intention of remaining a virgin forever. Luke is aware of this difficulty, but merely notes the situation without offering any explanation. The fact that the Evangelist, while stressing Mary's intention of virginity, also presents her as Joseph's spouse, is a sign of the historical reliability of the two pieces of information.