I: ... "why should anyone assume that Mary was a virgin until she died?"
Good question. The basic answer is that second-century ascetics invented tales picturing Mary as a perpetual virgin (The Protevangelium of James is perhaps the definitive example of this) and the idea found fertile soil in the minds of the fourth-century church as, in some circles, it embraced syncretistic idea borrowed from pagan religions -- such as the idea that leaders should be celibate.
I: "I'd imagine that in the situation, once Mary was ritually clean, her and Joseph became as any other man and wife."
Yup.
I: ... "Did Mary Mother of Jesus's parents have such limited imagination that they decided to name both of their daughters "Mary"?"
That's what the scenario which Jerome seems to be unavoidably implying.
I: "Or is it merely a curious attempt to insist that Mary was a virgin til death?"
That's what I'd say it was -- though Jerome was just defending an idea which he had inherited.
I: "And if so why?"
I can't mind-read Jerome and his contemporaries, but the authors of the earlier works on which the idea is based (again, the Protevangelium of James comes to mind) probably invented the idea to promote asceticism and celibacy, which they cherished for reasons completely unrelated to the Biblical text.
Yours in Christ,
Waterrock


