I think your point is best made with Matt 1:25, and did not know her till she brought forth her son -- the first-born, and he called his name Jesus. It makes no sense that Mary became infertile.
No sense to whom, you?
Matthew 1 says nothing about Mary's relations with Joseph after Christ was born. The firstborn is a firstborn even if he remains the only male child, it is a legal status. "Till", Greek "eos", is often used in the sence that refers strictly to the past without reference to the future, like in "the field was called Haceldama, that is, The field of blood, [eos] this day" (Mt 27:8).
In the economy of salvation, Mary giving birth to others would give a dubious status of Brother of God to the putative sibling; it would also signal that with mothering Jesus her work was somehow not gloriously complete. It would also conflict with her adoption of, and by, St. John (John 19).