The scriptures tell us: “For ALL have sinned and fallen short of the glory of God.”
Notice it doesn’t say “except for Mary.”
The existence of the Christ who is a wholly Divine member of the Godhead does not require a mother born of a virgin... logically or structurally. Mary’s holiness did not come from a sinless life or an immaculate conception but from the same place ours does: through a belief in the Godhead and faith in the redemptive work of the God. The source of her holiness was not of herself, but was imputed unto her via Christ.
As for the supposed double standard of womens’ purity and sexuality, where’s the rub? A chaste woman who remains pure and modest before the world is not defiled in one iota by a wanton and expressive marriage bed. How else does a man and woman become “one flesh?”
structurally = scripturally
Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. This is indicated by Paul later in the letter to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11).
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So if Paul’s statement in Romans 3 includes an exception for the Jesus, one may argue that an exception for Mary can also be made.