http://www.catholic.com/tracts/brethren-of-the-lord
Consider this line: "Michal the daughter of Saul had no children till the day of her death" (2 Sam. 6:23). Are we to assume she had children after her death?
There is also the burial of Moses. The book of Deuteronomy says that no one knew the location of his grave "until this present day" (Deut. 34:6, Knox). But we know that no one has known since that day either.
The examples could be multiplied, but you get the idea—nothing can be proved from the use of the word "till" in Matthew 1:25. Recent translations give a better sense of the verse: "He had no relations with her at any time before she bore a son" (New American Bible); "He had not known her when she bore a son" (Knox).
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The first male child of a marriage was termed the "first-born" even if he turned out to be the only child of the marriage.