Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost . . . And [Joseph] knew her [Mary] not till she had brought forth her son: and he all his name Jesus.
(I have removed “firstborn” [Greek: πρωτοτοκος] from v.25 as it is a later interpolation, perhaps based on Luke 2:7).
When examining the biblical teaching on Mary and the question of whether she was a perpetual virgin, Matt 1:18, 25 are two key passages, as they show the impossibility of any other interpretation than the “Helvidian” perspective (viz. Mary and Joseph, after the birth of Jesus, engaged in sexual activity as husband and wife, and sired children after Jesus).
The phrase “before they came together” is πρὶν ἢ συνελθεῖν αὐτοὺς. The verb translated as “to come together” is συνερχομαι. While it is correctly noted by some Catholic scholars and apologists (e.g., John McHugh, The Mother of Jesus in the New Testament [1975]) that this phrase can mean “to come together [platonically]” or even “to live together,” this is not the contextual meaning of this verb; indeed, Matthew in vv.18-25 is defending the concept of the virginal conception and birth of Jesus, so a sexual/euphemistic meaning of this verb is in view here. That this is the case can be seen in how leading scholarly Greek lexicons understand this verb in light of v.18.
For instance, Louw-Nida in their lexicon (23.61) offers the following meaning of the verb in light of γινωσκω in v.25:
συνέρχομαι: πρὶν ἢ συνελθεῖν αὐτοὺς εὑρέθη ἐν γαστρὶ ἔχουσα 'before they had sexual intercourse, she was found to be pregnant' Mt 1.18
Moulton-Milligan in their Greek lexicon offer this definition of the verb in light of the papyri evidence:
The verb is common in connexion with marriage, as in Mt 118, e.g. BGU IV. 10506 (time of Augustus) συγχωροῦσιν Ἰσιδώρα καὶ Διονύσιος συνεληλυθέναι ἀλλήλοις πρὸς γάμο(ν), so ib. 10988 (c. B.C. 20), 11058 (c. B.C. 10), P Tebt II. 3512 (ii/A.D.) οἰκί(ας) . . δοθείσης αὐτῇ … συνερχο(μένῃ) τῷ ὁμομητ(ρίῳ) ἀδελ(φῷ), “a house given to her on her marriage with her brother on the mother’s side,” ib. 3344 (A.D. 200–1) σ[υ]νῆλθον πρὸς γάμον Ἑρμῇ, “I was united in marriage to Hermes” (Edd.): cf. also Gnomon 47 (= BGU V. p. 23) (c. A.D. 150) ἀστὴ συνελθοῦσα Aiv[γ]u[πτίῳ] kατ᾽ ἄγνοιαν ὡς ἀστῷ ἀνεύθυνός ἐστιν, and the use of the subst. συνέλευσις in PSI V. 45010 (ii/iii A.D.), with which the editor compares P Oxy XII. 14736 (A.D. 201) ἅμα τῇ τοῦ γάμ[ο]υ αὐτῶν προσελεύσει.
There is no hint that Joseph and Mary’s relationship was one of an aged protector and an avowed temple virgin, as later (mainly Eastern) or any other relationship but a genuine husband/wife relationship. This is further substantiated by vv.16 and 20, where the terms “her [Mary’s] husband” is used of her relationship with Joseph (Greek ανηρ) in v.16, and Mary is called Joseph’s “wife” (Greek: γυνη) in v.20.
The verb “to know” in v.25 is γινωσκω. While, like its Hebrew equivalent ידע it can mean intellectual knowledge, both terms carry the euphemistic meaning of “sexual intercourse,” similar to συνερχομαι in v.18 (e.g., Gen 4:1, 25 in both the Hebrew OT and LXX). Again, as this whole pericope (vv.18-25) is a polemic in defense of the miraculous conception of Jesus, the euphemistic meaning is clearly in view here.
Friberg, in his Greek Lexicon, offers the following definition in light of Matt 1:25 (emphasis in original):
[E]uphemistically have sexual intercourse (MT 1.25).
Jerome, in his work in defence of the perpetual virginity of Mary from A.D. 383, "Against Helvidius," understood the term to have such a meaning, although he tried to argue for a continuation of the celibate relationship between Mary and Joseph post-partum.
As Murray Harris in his book, Prepositions and Theology in the Greek New Testament: An Essential Reference Resource for Exegesis (2012) noted, had Matthew wish to convey the idea of a continuation of the abstinence post-partum between Mary and Joseph, the term “until” (Greek: εως ου) would not have been used on its own, as the predominant meaning of this phrase is “until ‘x’ but not afterwards” (i.e., a cessation of the main clause [see this post responding to Tim Staples on εως ου]) but would have coupled it with the Greek phrase η απο τοτε ("or after that" [i.e., the birth of Jesus]); however, he did not, and coupled with the texts in Matthew that speak of the “brothers” and “sisters” of Jesus (Matt 12:46-47; 13:55), the defender of Mary’s perpetual virginity is in a precarious position, exegetically.
Defenders of Mary's perpetual virginity are in the unenviable position of having to argue for a position that is totally at odds with the prima facie reading of the pertinent texts, as well as the exegetical, grammatical, and lexical evidence supporting the "Helvidian" perspective.
Mary being called “Virgin” after the New Testament
An interesting “counter” offered by some Roman Catholic apologists (e.g., Gerry Matatics; Tim Staples) is that Mary was often called the “virgin” after the birth of Jesus. They reason that, if one had a bachelor uncle, one would cease to call them a “bachelor” after they got married, so, analogously, if Mary lost her virginity after the birth of Jesus, no one would have predicated the title, “virgin” upon her unless she was a perpetual virgin.
Apart from being an absolutely desperate argument void of serious biblical exegesis, it is void of logic, too. Notice, as one example, that John the Baptist was still called “the Baptist” even after he was killed by Herod, such as Matt 16:14 17:13. Why? Obviously, John was not pictured as baptising people in his posthumous state. The reason why John was called “the Baptist” post-mortem, and why Mary was called “the virgin” after the birth of Jesus, was due to the fact that these titles reflected their functions in the life of Jesus. There is no justification for reading into the predication of the term, “virgin” of Mary as evidence of perpetual virginity, especially as the term “ever-virgin” was not used in the earliest writings of the patristic era—such was a later development.
For Further Reading
Eric D. Svendsen, Who is My Mother? The Role and Status of the Mother of Jesus in the New Testament and Roman Catholicism.
John P. Meier, A Marginal Jew Rethinking the Historical Jesus, vol. 1: The Roots of the Problem and the Person
John McHugh, The Mother of Jesus in the New Testament