It does signify primacy of position, as seen in Colossians 1:15, 19; Rv. 1:5; and without being contrary to His Divine nature, spiritually The Son is called "the firstborn among many brethren" - Rom. 8:29, as well as Jer. 31: 9, in which Ephraim is called the firstborn, although he was the second one born (cf. Deuteronomy 21:15-17), however, unless stated otherwise, Firstborn normally does refer to the first Son among others. For TMK, “firstborn” never refers to any child who has or never had any physical siblings, at least from their father, which Issac had. If you want to search then there are 117 occurrences pf the masculine noun in the Hebrew Bible, translated “Firstborn.” and 9 in the NT for the Greek word.
And as for what a word "implies," "And knew her not till she had brought forth her firstborn son: and he called his name JESUS" (Mat 1:25) clearly implies a terminus and change or allowing for it, which is what the Greek word for "till" almost always indicates.
In addition, marriage is described from the beginning as sexual cleaving, (Gn. 2:24; Mt. 19:3-5) and there is only one possible exception to that, which was that of a very infirm old man, David, with Bathsheba. (1 Kings 1:1-4) which was due to the infirmity of the man
We also have texts which indicate Mary had other children. (Mat_12:46,48, Mat_27:56; Mar_15:40,47, Mar_16:1; Luk_24:10; Joh_19:25; Gal_1:19) Likewise, Psalms 69:8 states, "I am become a stranger unto my brethren, and an alien unto my mother's children. " Adelphos* (brethren) often refers to biological siblings, and the Greek has words it can use for cousins [or other kin] which are different words than “adelphos” (“suggenēs:” Luke 1:36, 58, 2:44, 21:16, 14:12; Mk. 6:4; Jn. 18:26; Acts10:24; Rom.9:3, 16:7, 11, 21; or “anepsios:” Col 4:10).
Neither is there any need for Mary to be a perpetual virgin, or to be sinless (any more than her parents) in order to bring forth Christ, being the vessel thru whom God provided a body, (cf. Heb. 10:5) than there was for the writers of Holy Writ to be sinless, by whom the pure written Word of God was brought forth. If Mary was sinless, then consistent with Rome's logic then her parents must have been sinless. Instead of restricting God, He manifests that He is not bound by Rome's requirements, and thus the genealogy of Mary included a harlot, and Israel itself is said to have brought forth Christ. (Rm. 9:4)
Which means that once again, at best, this Catholic tradition requires an exception to the norm, which is that brethren meant other children of Mary. And the Spirit characteristically records extraordinary aspects of even far less persons in Scripture, From extreme age (Methuselah), to excess size, fingers (Goliath), strength (Samson), speed (Asahel), sterility (Hannah), a celibate marriage (David and Abishag), prolonged celibacy (Anna), birth by a virgin (Mary), ascetic diet (John the Baptist), uncharacteristic singleness (Paul and Barnabas), and uncharacteristic duplicity of Peter, the surpassing grace and labor and suffering of Paul, the lack of genealogy of Melchizedek, etc., to Christ being sinless, which is mentioned at least thrice.
Meanwhile, since this abstinence would require the assent of her husband as the head, then if perpetual Marian virginity (PMV) was the case, and since this would normally be more difficult for the male, then Joseph should deserve more honor.
Here and here are a couple of supplemental response I provided to two RCs years ago, by the grace of God.
I do not argue that one must believe that Joseph and Mary did not remain celibate, but you simply do not have a Scriptural case for making Perpetual Marian Virginity a required belief. Resorting to Cath tradition only confirms the progressive accretions of traditions of men, and then teaching them for doctrines. Tragically.
Or evidence that it was believed from the beginning. Unless you can show that early Christians did not believe in the perpetual virginity of Mary you cannot assume that it was a later accretion.