With the exception of marriages concerning particular categories, such as the marriage of Samson, the Jews differentiate two periods in marriage: the agreement on the marriage and the marriage itself. The first period, the agreement on the marriage, is called betrothal or engagement.
Betrothal settled up the agreement between the two families. The two families were linked together by the payment of the mohar, which was a gift from the husband-to-be to the family of his “promised in marriage”. Probably this agreement was accompanied by a special feast. The woman “promised in marriage” wasn’t called yet a “wife”, but her status was nevertheless changed by this prior agreement. All infidelity was severely punished, because she was undermining established rights. Betrothal could last quite a long time, and in this case, the “promised in marriage” was exempt from military service.
Betrothal ended, either by the breach of contract between the two families (the mohar was reimbursed) or by the donation of the young woman to her husband, who normally took his new wife to his home. However, all the father had to do was to leave a room at his disposal where his son could stay from then on with his wife. It is undoubtedly at the time when the young woman gave up paternal protection that she would receive a benediction, along with blessings for her fertility.
Joseph and Mary’s betrothal can be explained according to these institutions. Mary had been promised in marriage to Joseph. He hadn’t taken her to his home, or more generally speaking they hadn’t lived together yet, when he realized she was expecting a baby. He had the legal right to breach the contract and even thought of doing so discreetly, but a divine intervention changed his mind.
Chanut (Père Christian Philippe Chanut)