From the WELS Q & A site:
Q#1: Did Mary have other children?
A: There are three theories about Jesus' brothers and sisters who are mentioned in the gospels. One is that these were actually Jesus' cousins. Another is that these were children of Joseph, whose first wife had died before he married Mary. Both of these theories were motivated at least in part by the desire to preserve Mary's virginity even after Christ's birth. The third idea is that these were children of Mary and Joseph born in a natural way after Christ's birth. This is the most natural understanding of the passages in which Jesus, Mary and these brothers and sisters appear together. See, for example, Matthew 12:46 and 13:55.
Q#2: To summarize, you stated that Mary did indeed have other children with Joseph and that this is "the most natural understanding of the passages." I find that rather hard to believe, though this oversexed society probably does not. Allow me to explain...
Both Mary and Joseph were no doubt amazed/shocked at her pregnancy when in fact they had had no sexual relations. The bible states that BOTH Joseph and Mary had an angel appear to them explaining the situation. Certainly the two them must have discussed these visions among themselves. Now imagine Joseph and Mary, BOTH pious Jews, realizing that God had chosen HER to give birth to His one and only Son -- The Word made flesh.
Are we then to assume that, pardon the crudeness of this comment, that both were just dying to jump into the sack together after they BOTH experienced the appearance of an angel followed by the miraculous birth of The Christ!?
Certainly they BOTH would have understood that Mary was "Blessed among women" and that she was in fact the Mother of God.
In your response (above) you imply Joseph was NOT going to respect that. Get real!
Now perhaps you will respond that is all a story and not really true. Well what else do you care to edit out (or mythologize) of the Bible (or Christ's teachings) that are critical of present day concepts of 'sanctity' or 'morality'?
If Scripture wished to make it clear that the brothers and sisters of Jesus were half brothers and sisters, this could have been done with only a few words. For example, in Matthew 1:25 Scripture could have said that Joseph never had any union with Mary instead of saying "He had no union with her until she gave birth to a son." This verse plus the fact that Scripture speaks matter of factly about Jesus' brothers and sisters certainly indicates that they could have been such.
To refer to Mary and Joseph as "jumping into the sack" after Jesus birth is indeed a crude comment. It seems to imply that all sexual union is an expression of sinful lust. Scripture speaks of the loving sexual union between a husband and wife as a blessing of God. For Mary to enjoy this blessing would not have been a sin for her, nor would it have affected in any way the fact of Jesus' miraculous birth.
It is only the false attempt to make of Mary something she is not that seems to drive the denial that Jesus could have had brothers and sisters. Mary herself says that she rejoiced in the birth of the Savior of all God's people among whom she includes herself (Luke 1:46-55). All Christians will honor Mary highly, just as we do other people through whom God accomplished great things such as Moses, Elijah, David, Paul etc. But like us, each of them rejoiced in the Savior by whom their sins were washed away and thus they were made heirs of eternal life.