Matthew 1:25 And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
It logically follows that he knew her AFTER.
And the text says Firstborn not ONLY.
No, it doesn't. Logic is completely silent on this.
The term 'Firstborn' is wreathed with legal importance in Hebrew culture, especially as regards inheritance. See e.g. Esau and Issac, mess of pottage and so forth. Firstborn doesn't mean 'First of many'.
First born also assumes there were others that followed.