Her Ever-Virginity means both her spiritual purity and her bodily virginity.
It is as Eikos 7 of the Akathists to Our Sweetest Lord says:
New was the creation which the Creator revealed to us when He appeared, for without seed He took flesh of a Virgin and rose from the tomb without breaking the seal of either, and bodily entered to the Apostles whilst the doors were shut.
How He could have been born by natural means *and* the Theotokos could stay a virgin in the bodily sense, is a mistery. But, as I say, He actually died and resurrected, He entered places with the doors shut... how dificult could it be for the Almighty to actually preserve her bodily virginity?