The Virgin Birth It is a matter of Catholic faith that Mary was a Virgin at the conception and at the birth of Christ, and that she always remained a virgin after the birth of Christ.
After Jesus was born Joseph did know his wife:
Matt 1:25 And knew her not till she had brought forth her firstborn son: and he called his name Jesus.
http://biblehub.com/interlinear/matthew/1-25.htm
"καὶ ouk ἐγίνωσκεν αὐτὴν ἕως οὗ ἔτεκεν υἱόν;"
ἕως is the greek preposition "until."
And this from the "Greek Expositors Bible":
"ἕως does not settle the question. It is easy to cite instances of its use as fixing a limit up to which a specified event did not occur, when as a matter of fact it did not occur at all. E.g., Genesis 8:7; the raven returned not till the waters were dried up; in fact, never returned (Schanz)."
There are number of commentaries discussing the use of the word until and this question.
http://biblehub.com/commentaries/matthew/1-25.htm
Good studying!
And it says “her FIRSTBORN” son, not her ONLY son.
1 Corinthians 7:5
Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
Hebrews 13:4
Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge.
Galatians 1:19
But other of the apostles saw I none, save James the Lord's brother.
Thank the Lord that Mary did not defraud Joseph and for James the Lord's brother the offspring of their Holy Matrimony!