What about the turn of phrase, "knew her not"?
"And knew her not till she had brought forth her firstborn son: and he called his name JESUS."Even the Vulgate says exactly the same thing and its pretty darned clear...
et non cognoscebat eam donec peperit filium suum primogenitum et vocavit nomen eius Iesum
Here's a link to a Latin to English translator, copy and paste and see what you get
Please enjoy this more thorough explanation by Tim Staples http://www.catholic.com/magazine/articles/the-case-for-mary%E2%80%99s-perpetual-virginity
Here are a few highlights:
Scripture’s statement that Joseph “knew [Mary] not until she brought forth her firstborn” would not necessarily mean they did “know” each other after she brought forth Jesus. Until is often used in Scripture as part of an idiomatic expression similar to our own usage in English. I may say to you, “Until we meet again, God bless you.” Does that necessarily mean after we meet again, God curse you? By no means. A phrase like this is used to emphasize what is being described before the until is fulfilled. It is not intended to say anything about the future beyond that point. Here are some biblical examples:
2 Samuel 6:23: And Michal the daughter of Saul had no child to (until) the day of her death. (Does this mean she had children after she died?)
1 Timothy 4:13: Until I come, attend to the public reading of scripture, to preaching, to teaching. (Does this mean Timothy should stop teaching after Paul comes?)
1 Corinthians 15:25: For he (Christ) must reign until he has put all his enemies under his feet. (Does this mean Christ’s reign will end? By no means! Luke 1:33 says, “he will reign over the house of Jacob forever and of his kingdom there shall be no end.”)