Mary is a type of the Church, and the Church is a virgin dedicating her life to her spouse, Jesus Christ.
Mary was not the "spouse" of Jesus, she was his mother. She was the spouse of Joseph and as such she had been commanded to cling to him, forsaking all others. If she did not cling to Joseph, then she was a sinner. If she gave all of her attention to Jesus and none to husband then she was a sinner.
Now you can avoid all this stuff by denying that Mary was really Joseph's wife, and that her suppposed marriage was simply a sham. Is that what you are implying?
The fruit of her womb is greater than all the fruit of all other wombs combined.
-A8
In a mystical sense, she IS! We see Mary as a type of Church, and Mary is our spiritual Mother, just as the Church is our Mother (giving birth to us through baptism and nuturing us through the Word and Sacrament). BOTH bring Christ to us. Mary has been given to us by Christ - He gave her to the beloved disciple. We are ALL His beloved disciple. Who was He giving to us? Mary/Church. Christ established a community through which His graces flow. Even you must admit that the Church, the people of God, has given us the Scriptures and proper understanding of it. Thus, we see that through the Church/Mary, all graces flow. We see the Church/Mary as a virgin, undefiled and wholly committed to Christ. We see obedience and faith in Christ come from Church/Mary. In Scriptures, the Fathers point to various places and see BOTH Mary and the Church - Song of Songs, Genesis 3:15, Rev 12, Luke 1. Really, you probably could make a case that anything with Mary in it can point to the people of God - and vice versus.
With this rough analysis, perhaps you can understand why the Church declares that Mary was a perpetual virgin. It says something about the CHURCH, as well as Mary.
Regards