On a recent Dividing Line there was a clip played taken from, if I recall correctly, the "Catholic Answers" show, regarding Joseph and Mary's "marriage." The caller asked (and I may be slightly paraphrasing) two questions:
1) Where in the Bible does it say that marriage is only valid when it is consummated?
2) Did Mary and Joseph have a valid marriage?
The host (well, the person providing the "Catholic Answers") answered the first question by appealing to Genesis, where it says that the "two shall be one flesh."
The host then went on to say that Joseph and Mary never became one flesh, but (and again I'm paraphrasing) that was ok because NT sacramental marriage hadn't come to be, yet. But if Genesis is the institution of the definition of marriage as valid depending on physical union, then the fact that sacramental marriage hadn't come to exist yet is irrelevant - since the question wasn't whether the marriage was sacramental, but whether it was valid.
They did have marriage before the apostles, and physical union was a normative aspect of Old Testament marriage. That's one reason that the New Testament places such great emphasis on the fact that Mary and Joseph didn't "know" each other from before Jesus was conceived until Jesus was born.
Matthew describes it as being that Mary was found to be with child of the Holy Ghost "before they [i.e. Mary and Joseph] came together" (Matthew 1:18). Of course, Catholicism today claims that Mary and Joseph never came together, but the natural sense of the text is that they did come together, just later. This is especially so when coupled with the statement, only a few verses later that Joseph "knew her not till she had brought forth her firstborn son," rather than saying that Joseph "never knew" her.