At the end of what does he say that he’s not imposing a rule on them?
Why is Mary’s alleged virginity such an issue for Catholics and how does it affect one’s salvation?
Because the blessedness of the Mary of Catholicism was not because of who she was privileged to carry and to mother by God's grace (though that is what the Mary of Scripture expressed), but it was due to her surpassing all creatures in holiness and virtues, and perpetual virginity is one of those virtues necessary for elevation to Godhood. You must remember reading that in Gnosticism 3:16. And in Acts 29 it was prophesied that this Mary would precede all other believers by being bodily resurrected and even crowned in glory, thus completing her near parallelization with Christ, as the divinized Mother of God.
Moreover, in detailing the many ways the New Covenant is "better" - the key word in Hebrews - this Mary is presented as the fulfillment of the ark of the covenant, who no man was to touch, just as she is set forth as our great high priestess, who was tempted in all points as we are yet without sin - including during nights with Joseph - and ever lives to make intercession for us, and by whom we have access into the holy of holies in Heaven. Therefore we are exhorted to come to her at the throne of grace to obtain mercy and help in time of need. (Hebrews 14:6-66) For all such "scripture" is given by the seducing spirit of Catholicism.
The above actually falls under the heading, "Proofs that the Catholic church did not change the Bible." but instead she just exalts mortals far far far above what is actually written.