The Blessed Virgin Mary's Immaculate Conception and Perpetual Virginity as Fitting Ad Extra Works of the Trinity, Co-Natural to the Ad Intra Act of God the Father's Generation of God the Son.

All the works of the Trinity ad extra within the economy of salvation are co-natural with the ad intra life of the Trinity. From the ad intra nature of the Trinitarian generation of the Son by the Father, it is fitting that God act through the Blessed Virgin Mary as an ad extra work of the Trinity, for Mary to imitate the ad intra life of the Trinity.

The Original immaculate Person is God the Father, who acts immaculately to generate the Son of God (the Word of God). For God the Father is most holy and every act the Father does is without sin. The generation of God the Son is from God the Father as the immaculate Father for the immaculate Son. Analogously, when the Son of God became flesh through the Blessed Virgin Mary, the immaculate Son of God was generated as from a mother, who acted in a way like that of God the Father. 

As God the Father acts as an immaculate Father of the immaculate Son, then analogously, it is fitting that the Blessed Virgin Mary also be immaculate to generate the immaculate Son of God. The immaculate Mary is Mary from the immaculate conception as a grace God gave Her to be the fitting mother of God the Son as an imitation of God the Father.

Also, God the Father acts to generate God the Son as a virginal act of generation of a divine person without the need for a sexual union. The Father's generation of the Son is an ongoing, and thereby a perpetual act of virginity of God the Father within the Trinity. Analogously, as the Blessed Virgin Mary generates God the Son, She acts in imitation of God the Father as an ad extra work of the Trinity, for Mary to act virginally to generate God the Son. As God the Father's generation of God the Son is an act of perpetual virginity, so too Mary was also a perpetual virgin in imitation of God the Father.

Conclusion - From the co-naturality of the ad intra life of the Trinity with the ad extra works of the Trinity, it is most fitting that the Blessed Virgin Mary received the grace of the immaculate conception and was a perpetual virgin for the sake of the Son of God.