The Perpetual Virginity of Mary has been taught from Apostolic times, and recorded since the second century in Christian writings, and was universally held among all Christians until 1500 years later. That Mary was not a Perpetual Virgin is a new false gospel that comes with the heresies attendant with protestantism.
It is you who proclaim a new false gospel. Therefore the onus is on you to prove from scripture that Mary had relations with Joseph after the birth of Christ.
That you cannot do.
I can do anything I want, and so can you - including believing that sex between a husband and wife somehow lessens the stature of the wife, and that there’s a queen of Heaven.
Thanks for at least finally admitting that the basis of your argument is appeal to authority.