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'.
Come on now...you know you can't just rely on the Bible for these things. I mean, surely, some saint somewhere down the road heard this from someone else who thought that it meant something and so on and so on.
That doesn’t mean that they had relations. Mary was a perpetual virgin. Even Luther said so.