Don’t aim for the straw man, go for the flesh and blood one aiming a lance at you.
David, blogging at the Thirsty Theologian, aims at the Roman Catholic dogmas on the Theotokos (I’m guessing the title is one to which he’d admit she has claim). Now … I’ve only got half a dog in this fight, as the Eastern and Roman beliefs about the Theotokos differ somewhat, although their overlap is far more than of that with the Evangelical. I’m not going to claim she never sinned or that she was immaculately conceived. But still …
The point is making silly claims about marital duty/bed and so on is irrelevant, consider 1st Kings 1:1-4. The teaching is that Joseph was elderly when he married a very young Mary and never had relations with her, as this was just a few centuries before the population was sex obsessed and email spam about little blue pills became inescapable. Tradition holds as well that the brothers of Jesus were by Joseph’s first wife. David might also wonder, or confront, why both John Calvin and Martin Luther affirmed that she remained virgin throughout her life. These were and are not inconsequential protestant theologians
I will not attempt to give all the reasons and argument for why her continued virginity is believed, but just one might be exemplary. In our recent feast of the Dormition (the Roman feast of the Assumption) we read this Old Testament lesson (Ezekiel 43:27-44:4). Why? The Theotokos is a type of the Mercy seat (by her womb where God “sat”). By bearing Christ she became a symbol/type of the mercy seat or the holy of holies. This door of which we read in Ezekiel, “shall remain shut, it shall not be opened.” and so on. I’m hoping that the clear connection that can be made from that to her virginity is clear. This is not the only Scriptural reference which the early church used which points clearly to her virginity.
The early church affirmation of her continued virginity prior to and after her birth were based on this and testimony (oral not in Scripture) traced to Luke who tradition holds has the Mary/Virgin story on account via an extended interview he had with her prior to her Dormition (falling asleep).
Look, it’s easy to fall into the trap of arguing against straw men instead of the more difficult task of addressing their actual teaching and beliefs. I’m pretty sure I fall into that trap all the time. So, don’t be shy about pointing it out when I do it and I’ll continue to do it when I see it as well. I would recommend if you want to tilt against Roman’s that this should be a required reference: Catechism of the Catholic Church, Alas, if you want to tilt against the East there is no such thing.