Even as a Catholic, it's perplexing how remaining a virgin in marriage can be considered a virtuous wife.
The way the early Christians believed was that Mary was devoted to God, jsut as Samuel or Anna (Luke 2:36-37) were and that Joseph was already old at the time of the marriage and he an elderly man, a widower to be her protector and spouse
As the protector of this consecrated virgin of God he performed his duty and a devoted father to this strange child born to a virgin.
As a person consecrated to the service of God, Mary would not have had sexual relations post this
Finally, it's a niggling point, but the Gospel does not actual mention their marriage -- did he or didn't he? We know Joseph was a good protector and father, but yet years later, Jesus is called the son of Mary, not the son of Joseph. Even if Joseph were dead, wouldn't they call Jesus the son of Joseph (even an adopted one?)
Do note also that this does not date only to 400 AD but to earlier, the 2nd century. You can find references as well in Origen (Ad 248)
If you want to be precise, at the best one can state it is ambiguous but something that has been believed since the time of the Early Christians