All ancient Christian Tradition highly respects virginity, and virginity for the sake of God's Kingdom especially. All who claim to be Christians generally accepts Our Lady's virginity at the birth of Christ. In the recent years some Christian denominations tend to denigrate the value of virginity lived for the sake of the Kingdom (married clergy, homo clergy, lesbian clergy). IMO, it is the very mindset from which they also denounce perpetual virginity of Our Lady.
Is it important to the faith? Yes, it is, just like our belief in the Communion of Saints, in Purgatory, in one holy, Catholic and Apostolic Church, primacy of the Bishop of Rome etc.
There is a saying: "He who is not Marian is Arian," meaning, without proper understanding and due respect for Mary one would be unable to grasp the divinity and the humanity of Christ -- rejection of Mary as an ever-Virgin Mother of God leads to a very deficient, or even heretical understanding of Christ our God and Savior.
If a Catholic did not believe in Mary's perpetual virginity, would that prevent the Catholic's salvation?
The question could be paraphrased, how much of the Catholic teaching can one reject (or not accept) without jeopardizing one's eternal Salvation?
I hope we both know the answer :-D