I have no strong opinion about this, but it seems to me that Matthew 1:25 at least suggests it. It would be odd to say that Joseph refrained from having sex with Mary until after the birth of Jesus, if he continued to refrain afterward as well. If he never consummated the marriage, then that's what it would make sense to say.
Then Joseph her husband, being a just man, and not wanting to make her a public example, was minded to put her away secretly. But while he thought about these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not be afraid to take to you Mary your wife, for that which is conceived in her is of the Holy Spirit. And she will bring forth a Son, and you shall call His name Jesus, for He will save His people from their sins.”
So all this was done that it might be fulfilled which was spoken by the Lord through the prophet, saying: “Behold, the virgin shall be with child, and bear a Son, and they shall call His name Immanuel,” which is translated, “God with us.”
Then Joseph, being aroused from sleep, did as the angel of the Lord commanded him and took to him his wife, and did not know her till she had brought forth her firstborn Son. And he called His name Jesus.
Matthew 1:19-25
Note also how it refers to Jesus specifically as Mary's "firstborn" - not 'only born'.
Exodus 13:13
“Redeem with a lamb every firstborn donkey, but if you do not redeem it, break its neck. Redeem every firstborn among your sons.”
So are we saying we know the firstborn donkey had brother and sister donkeys?? Or was that how the language was used?
” The “first-born of the poor” signifies the most miserable of the poor ( Isaiah 14:30 ). The “church of the first-born” signifies the church of the redeemed.
The destruction of the first-born was the last of the ten plagues inflicted on the Egyptians ( Exodus 11:1-8 ; Exodus 12:29 Exodus 12:30 ). “
http://www.biblestudytools.com/dictionary/first-born/
Maybe First-Born has a rather expanded meaning.