Brad
Mary is the new Ark of the Covenant. She must remain undefiled just as the Ark had to be undefiled.
I am certain that you remeber in the old Testament that when the Ark was being transfered and one of the priests put his hand on it to steady it that he died instantly.
The same would be true of Mary for her and Joseph to have relations would have defiled her.
Please correct me if I'm wrong, but it wasn't a priest who died while steadying the Ark. I think the priests (from the tribe of Levi?) were the only ones who could handle the Ark.