The Book of Numbers and the Perpetual Virginity of Mary
Singing In The Reign: A Biblical Basis For Mary's Perpetual Virginity? - a discussion of Numbers 30:3-15:
Now, what this means is that if a young Jewish woman--say, Mary, in this instance--took a vow of sexual abstinence, and her legal husband--in our case, Joseph--heard of the vow and said nothing, then the vow stands, and she is bound to keep it. This provides a solid historical basis for Joseph and Mary having a perpetually virginal marriage: indeed, Numbers is very explicit in the final verse that if the husband changes his mind "and makes them null and void after he has heard of them," the the sin will be upon him: "he shall bear her iniquity" (Num 30:15). One can easily imagine a situation where some husbands would think better of deciding to accept such a vow! But as Matthew's Gospel tells us: Joseph was a "righteous man" (Matt 1:19), and obedient to Torah. If Mary took a vow of sexual abstinence--and her words "How can this be, since I know not man?" in Luke are evidence that she did (Luke 1:34)--and if Joseph accepted this vow at the time of their wedding, then he would have been bound by Mosaic Law to honor her vow of sexual abstinence under the penalty of sin.
Hat Tip: Catholic and Enjoying It!