Biblical Proof that Mary did not Remain a Virgin

There are at least 3 Biblical reasons that Mary did not remain a virgin and had other children after Jesus that are rarely mentioned. Everyone talks about how Joseph only kept Mary a virgin until Jesus was born and how Jesus had brothers and sisters mentioned in scripture. Those are good arguments. However, there are even stronger arguments that should be mentioned.

1) If Joseph had children before Jesus was born, then Jesus was not the rightful heir to the Davidic throne and could not have been the Messiah. Matthew 1 shows the kingly lineage through David all the way down to Joseph. Jesus was Joseph's legal firstborn son, and thus the rightful heir to the throne. If Jude or James or one of Jesus' other brothers was actually an older son of Joseph, he would be the rightful king instead of Jesus.

2) Sex is and has always been an integral part of what marriage is. From the very beginning in Genesis, God defined marriage as a man joining to his wife and they become one flesh. We refer to sex after the marriage ceremony as consummation because it completes the marriage. If Mary did not become one flesh with Joseph, she wasn't married to him. The union was never completed. Yet scripture tells us she was his wife. Catholics are thus arguing that there can be such a thing as a sexless marriage which is never completed with a one flesh union. This is unbiblical, not only about Mary but about marriage itself.

3) The Old Testament specifically prophesies that the Messiah will be rejected by His mother's sons. If Mary had no other children, then this prophecy failed and Jesus was not the promised Messiah.

Psalm 69:7-9
For it is for your sake that I have borne reproach, that dishonor has covered my face. I have become a stranger to my brothers, an alien to my mother's sons. For zeal for your house has consumed me, and the reproaches of those who reproach you have fallen on me.

We know this is a Messianic passage. In fact, verse 9 is specifically quoted in John 2:17 as referring to Jesus when He cleansed the temple. Yet verse 8 is only fulfilled if Jesus had brothers who were specifically sons of Mary who then rejected Him. Scripture records their rejection. They believed Jesus was crazy initially. It was not until He rose from the dead and appeared to them that they believed. These must have been sons of Mary, as scripture foretold.

There's also no reason that Mary needed to remain a virgin after Jesus was born. The sign had already been given. Mary's virginity at the birth of the Messiah was a sign about Jesus, not about Mary.