A question was put to me recently over the validity of Joseph and Mary's marriage in light of the Catholic understanding that the two remained chaste for the duration of their marital relationship. This is a question I hear quite often from both non-Catholic Christians and even from fellow Catholics. The assertion they make is this: if their [Joseph and Mary's] marriage was not consummated, then it was not a valid marriage and would not have been acceptable given Jewish laws and customs.
First, some things to consider regarding Mary's perpetual virginity:
a.) If Mary had other children, then where were they or why weren't they mentioned when Jesus was lost for three days in Jerusalem at the temple during Passover (Luke 2:39-52)?
b.) When Jesus hung on the crucifix with Mary, his mother, and the "disciple whom he loved" (commonly accepted as John the apostle) and charged this disciple with the responsibility of taking Mary into his own home (John 19:26), if Mary had other children, then why wasn't this responsibility issued with them? If such children existed, then this would have been a grave offense as it was customary for Jewish parents to be taken into the homes of their own children (usually the eldest child).
c.) Why is Jesus referred to as "the son of Mary" and not "a son of Mary" in Mark 6:3?
Yes, "brothers" of Jesus are also mentioned in this passage (James, and Joseph, and Jude, and Simon; and sisters), but it may have also been the case that as Joseph was considerably older than Mary, he very well may have had children from a previous marriage; he may very well have been widowed (a document called the Protoevangelium of St. James backs this claim). So any "siblings" of Jesus were likely to be step-brothers or step-sisters. They very well may as well have been cousins, but since there was no word in Hebrew or Aramic for "step-brother/sister" or "cousin", "kin" or "kinsfolk" would have been the word used to describe the relationship Jesus had with these others. The use of the word "brothers/sisters" has been used errnigly in modern translations of the Bible and this may be due to shoddy translating.
It could be suggested that Joseph himself not being a virgin is somehow "disrespectful" to Mary and in their marriage, and this is where we have to distinguish between "chastity" and "virginity" and that even in not being a virgin, the purity of the marriage can be upheld.
Chastity is not the same as virginity, that should be very clear in most minds already. Chastity pertains to a self-discipline whereas virginity pertains to one's sexual state of being. One can be chaste in a relationship having had a previous sexual encounter, i.e. in the case of Joseph and Mary, even if Joseph had a wife and marriage previous to being betrothed to Mary, it does not mean that he could not remain chaste in his relationship with Mary. Having certain sexual desires does not mean that they will be given in to.
On the matter of consummation and and the validity of marriage. Mary and Joseph were validly married. What consummation does is make the marriage indissoluble. Their marriage was already valid, but due to it not having been consummated it could still be dissolved. This is why Joseph when first hearing news of Mary's pregnancy considered divorcing her "quietly" so as not to shame her:
"and her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly." - Matthew 1:19
Divorce - this scenario - would not have been possible if the marriage was consummated; it would have been indissoluble.
I recommend reading the last few title points of this article for a more thorough explanation, but the key thing to focus on here is that a marriage is still valid even if it has not been consummated. Further to the point: any marriage is still valid even if it has not been consummated.