Justification of perpetual virginity of Mother Mary How do Catholics justify St. Joseph not being able to consummate his marriage with his wife? 

It would seem as though the doctrine of Mary's perpetual virginity is at odds with Catholic teaching about the purpose of marriage and holding the Holy Family up as the model for spouses would produce very few children, yet Catholics often produce very many children.

So why was St. Joseph's marriage destined to be devoid of marital intimacy?
I am not a Catholic so I guess I cannot give you the best answer, but here's my answer from an Eastern Orthodox viewpoint, which might be quite different from the Catholic one (or not?):

The ultimate purpose of marriage is actually the salvation of the bride and groom together. Bearing children is not a purpose in itself, but a consequence. Having children is encouraged but not 'mandatory'. Not bearing children was seen as a damnation in the Old Testament, but nowadays Orthodox fathers teach that it is fine if married couples choose to live together, but in chastity. See the works of Romanian Orthodox Fr. Arsenie Papacioc.