Jamieson, Fausset and Brown on Matthew 1:25

25. And knew her not till she had brought forth her first-born son: and he called his name JESUS--The word "till" does not necessarily imply that they lived on a different footing afterwards (as will be evident from the use of the same word in 1Sa 15:35; 2Sa 6:23; Mt 12:20); nor does the word "first-born" decide the much-disputed question, whether Mary had any children to Joseph after the birth of Christ; for, as Lightfoot says, "The law, in speaking of the first-born, regarded not whether any were born after or no, but only that none were born before." (See on Mt 13:55, 56).