Why did Mary need to remain a virgin her whole life?

I know that the Virgin Mary remained a virgin her whole life, which is why we refer to her as “ever-virgin.” My question is why. I understand that it was crucial that she be pure in order to bring forth Jesus, but what about after his birth? She and St. Joseph were married, after all, so it wouldn’t be sinful.

Virginity is a sign of dedication to God. It also frees a person to be available to all in a way that would not be possible if that person was involved in an exclusive relationship. By remaining ever-virgin, the Blessed Mother more completely dedicated her life to God, and that dedication became a sign of the spiritual fecundity of chastity by allowing her to become spiritual Mother to all those for whom her Son died.

Recommended reading:

Divine Maternity and Perpetual Virginity by Father Mateo
Refuting the Attack on Mary by Father Mateo
First Guardian of the Faith by Mark P. Shea

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