The passage you cite in Matthew, if read in context with the other passages involving His brothers and sisters, clearly indicates Joseph kept Mary a virgin until Jesus was born.
The verse in Matthew 1:25 for context:
but kept her a virgin until she gave birth to a Son; and he called His name Jesus. NASB
And he knew her not till she brought forth her firstborn son: and he called his name JESUS. Douay-Rheims
The word to focus on is "knew" or "kept". The Greek is ginosko. It has the meaning of to know, especially through personal experience.
Now we know Joseph and Mary knew each other. They had met so it doesn't have that meaning.
For additional context, as we are discussing Joseph and Mary's intimacy, refer to Luke 1:34 where Mary asked Gabriel, "How will this be since I do not know (ginosko...same word in Matthew) a man?"
Mary asks this question right after Gabriel had told her she was going to have a child.
As Mary was a virgin, which we all agree on, she was asking Gabriel how she would be a mom when she had not had sexual intercourse.
Mary was a good girl and had kept herself pure.
The passage in Matthew, if read in context with Luke, can only indicate one thing....Joseph kept Mary a virgin until Jesus was born.
After His birth, Joseph and Mary "knew" each other as married couples do.
If read in context.... after that they consummated the marriage producing other brothers and sisters.
we see in Colossians Col 1:15 who is the image of the invisible God, the firstborn of all creation;
Just so we have the verse for context.
He is the image of the invisible God, the firstborn of all creation. Col 1:15 NASB
Who is the image of the invisible God, the firstborn of every creature: Col 1:15 Douay-Rheims
The Greek word for firstborn in this passage is prototokos. It is an adjective. The definition means first born, eldest; the first among others.
Jesus is referred to as the first Born of God?
Actually in this passage He is the first born of all creation.
How many other children did God have?
Jesus is the only begotten, (GR: monogenes), Son of God as used in John 1:14,18; John 3:16,18; Hebrews 11:17; 1 John 4:9.
There is no contradiction of the text to call Jesus the only begotten Son of God, which He is, and to say He was the first born of Mary, which He is.
The Greek, along with context, indicates Joseph and Mary had sexual relations producing other children.
bookmarked
Luke 2:41-52 The Boy Jesus at the Temple 41 Every year Jesus’ parents went to Jerusalem for the Festival of the Passover. 42 When he was twelve years old, they went up to the festival, according to the custom. 43 After the festival was over, while his parents were returning home, the boy Jesus stayed behind in Jerusalem, but they were unaware of it.
44 Thinking he was in their company, they traveled on for a day. Then they began looking for him among their relatives and friends. 45 When they did not find him, they went back to Jerusalem to look for him.
46 After three days they found him in the temple courts, sitting among the teachers, listening to them and asking them questions. 47 Everyone who heard him was amazed at his understanding and his answers.
48 When his parents saw him, they were astonished. His mother said to him, “Son, why have you treated us like this? Your father and I have been anxiously searching for you.â€
49 “Why were you searching for me?†he asked. “Didn’t you know I had to be in my Father’s house?â€[a] 50 But they did not understand what he was saying to them.
51 Then he went down to Nazareth with them and was obedient to them. But his mother treasured all these things in her heart. 52 And Jesus grew in wisdom and stature, and in favor with God and man.
Where were the other children? The text tells us that Jesus was twelve years old He would have been traveling with the adult males preparing for his Mitzvah. The next oldest child would have been at best around age 10 and would have been traveling not just with the women, but directly with Mary.
No mention is made of these children being on the journey, left with relatives while Mary and Joseph searched or on the return journey.
You also failed to address as to whether Jesus will reign after His enemies are at His feet or how Many children Michael had after her death.
And the biggest points you ignore is that the word "until/ till" does not require that the condition change and the first born does not require that there be a second born. Ishmael was the first born of Abraham and Hagar, but there Is no mention of any other children by Hagar. Isaac is the first born of Sarah, but she had no other children.
The quote from exodus makes it clear "Firstborn" refers to the child that opens the womb.
Stop grasping at straws.