Only that the same authority which proclaims the faith, which wrote and canonized scriptures has also declared that Mary is perpetually virgin. Tear the cloth and it all unravels.
But, consider: Mary was betrothed, preparing for marriage. She knew how children were created (“knowledge of man”) in the carnal sense. With all of this in mind, she dared to ask Gabriel exactly how this Christ-child was to be conceived. See incongruity there?
We must also pay close attention to the words of Gabriel, as they are God’s words as expressed through His messenger. “You are to have a Son” (singular). What occurred? She did have that Son.
Look up brothers and sisters in the scriptures and you will see that entire communities, entire tribes of Israel are brothers and sisters. Those of the same geographic area are especially known as brothers and sisters. If you are in a labor union, what do you call the others? That’s correct: brothers and sisters.
Further, she declared herself to be the “handmaid” or “bond slave” of the Lord - her own Child! Whom else could she serve? As well, the recently concocted “brothers and sisters” canard forces a 19th-21st century template over first century Judaism and Hebraic culture.
Let’s turn this argument around on you: So what if Jesus was not God’s Son? He was a great teacher and left the world a wonderful legacy. He’s right up there with Buddha, Plato, Aristotle and the others.