PERPETUAL VIRGINITY - Mary is the New Testament type of the Ark of the Covenant (see obvious parallels in chart here
). The Ark was not to be touched (1 Chr 13:10
Another way to see it is Mary's response to the angel who told her she would bear a son. "How can I since I don't know man?" she says. Well, since she was about to be married, the obvious answer to her would have been because she'd bear Joseph's son. But if she had taken a vow of chastity, as some Jewish women did, then her question makes sense.
There are others who believe that the way Jesus entered the world (her womb) will never be crossed again by man:
Ezekiel 44:2 And the Lord said to me: This gate shall be shut, it shall not be opened, and no man shall pass through it: because the Lord the God of Israel hath entered in by it, and it shall be shut.
The "brothers" mentioned in Scripture are not His fraternal brothers
There's even more you could get into of Mary as the type of the Church presented spotless to the Lord, etc...