I'd love to hear from our catholic friends their response to this question.
I would think it would be a simple yes or no reply.
(Incidentally, it was also held by the founders of the Protestant faiths of Anglicanism, Methodism and Lutheranism: Luther, Latimer, Cranmer, Wesley, etc.)
So, to formally reject this is heresy, and involves rejecting the teaching authority which Christ guaranteed to His Church.
And that's a biggie. If you don't accept the virginity of Mary on the authority of the Church, why would you accept the doctrines of the Incarnation, or the Trinity, Baptism or the Lord's Supper or even the canon of Scripture on the authority of the Church? If you reject Christ's guarantees to His Church in principle, you've rejected what He constituted as the pillar and foundation of the Truth.
Whether one can be "saved" while rejecting the Perpetual Virginity of Mary, is another question. If one is in the mortal sin of heresy, no.
However, I don't think most theological dissenters are formal heretics. This is because they lack --- I think --- either the degree of knowledge or the degree of intentionality which would have to be there, to constitute a mortal sin. Denial of Mary's virginity is a material heresy ----an error --- but most who have fallen into this error don't reject the foundation of Truth formally, knowingly and contumaciously. That is, they are not willfully and obstinately disobedient to Christ.
At least, so I must assume in charity.