I have written on the perpetual virginity of Mary, including Matthew 1:18, 25 and the Perpetual Virginity of Mary, where I show that lexicons define the verb "to come together" (συνέρχομαι) in Matt 1:18 is used as a euphemism for sexual intercourse. Further evidence for this can be seen in BDAG (emphasis added):
As is often the case, the Roman Catholic apologist faces an uphill battle against sound exegesis and biblical scholarship to support her man-made dogmas.
7073 συνέρχομαι
• συνέρχομαι (since Il. 10, 224 [in tmesis]; ins, pap, LXX, pseudepigr., Philo, Joseph., Just.; Ath., R. 23, p. 77, 8) impf. συνηρχόμην; fut. συνελεύσομαι; 2 aor. συνῆλθον (συνῆλθα B-D-F §81, 3; W-S. §13, 13; Mlt-H. 208); pf. ptc. συνεληλυθώς; plpf. 3 pl. συνεληλύθεισαν, ptc. fem. pl. συνεληλυθεῖαι.
1. to come together w. others as a group, assemble, gather συνέρχεται ὁ ὄχλος Mk 3:20. συνέρχονται πάντες οἱ ἀρχιερεῖς 14:53.—Ac 1:6 (s. B-D-F §251; Rob. 695); 2:6, 37 D; 5:16; 10:27; 16:13; 19:32; 21:22 v.l.; 22:30; 28:17; 1 Cor 14:26. W. the addition of εἰς w. acc. of place (Pla., Leg. 6, 13, 767c; Diod. S. 13, 100, 7 συνῆλθον εἰς Ἔφεσον; Zech 8:21) Ac 5:16 v.l.; ἐν w. dat. of place (POxy 1187, 6) ἐν ἐκκλησίᾳ 1 Cor 11:18; αὐτοῦ Mk 6:33 v.l.; ὅπου J 18:20; ἐπὶ τὸ αὐτό (s. αὐτός 3b; σ. ἐπὶ τὸ αὐτό Josh 9:2; Syntipas p. 75, 16) 1 Cor 11:20; 14:23; B 4:10. Foll. by a dat. come together with someone, assemble at someone’s house (PTebt 34, 4 [I BC] συνελθεῖν Ὥρῳ; Jos., Bell. 2, 411) Mk 14:53 v.l.; D 14:2. πρός τινα come together to (meet) someone (Ex 32:26) Mk 6:33 v.l. Foll. by an inf. of purpose Lk 5:15; by εἰς denoting purpose 1 Cor 11:33; IEph 13:1. εἰς can also introduce a result that was not intended οὐκ εἰς τὸ κρεῖσσον ἀλλὰ εἰς τὸ ἧσσον συνέρχεσθε you hold your meetings in such a way that they turn out not to your advantage, but to your disadvantage 1 Cor 11:17. εἰς κρίμα vs. 34 (on the solemnity of a celebration cp. the schol. on Aristoph., Pax 967f: to the question ‘τίς τῇδε;’ [=``who is here?"] the group answers ‘πολλοὶ κἀγαθοί’. τοῦτο δὲ ἐποίουν οἱ σπένδοντες, ἵνα οἱ συνειδότες τι ἑαυτοῖς ἄτοπον ἐκχωροῖεν τ. σπονδῶν=``many good men." Now, those who pour the libations were accustomed to do this [ask the question], so that those who were aware of anything inappropriate about themselves might absent themselves from the libation). W. indication of the nature and manner of the meeting συνέρχεσθε ἐν μιᾷ πίστει IEph 20:2.
2. to come/go with one or more pers., travel together with someone (BGU 380, 13; 596, 4 [84 AD]) τινί (EpArist 35; Jos., Ant. 9, 33) τοὺς συνελθόντας αὐτῇ Ἰουδαίους J 11:33. ἦσαν συνεληλυθυῖαι ἐκ τῆς Γαλιλαίας αὐτῷ Lk 23:55. Cp. Ac 1:21; 9:39; 10:23, 45; 11:12. σ. τινι εἰς τὸ ἔργον 15:38. σύν τινι instead of the dat. alone 21:16. συνελθόντων ἐνθάδε prob. means (because of συνκαταβάντες 25:5) they came back here with (me) 25:17.
3. to unite in an intimate relationship, come together in a sexual context (X., Mem. 2, 2, 4; Diod. S. 3, 58, 4; Ps.-Apollod. 1, 3, 3; Philo, Virt. 40; 111; Jos., Ant. 7, 168; 213) ἐπὶ τὸ αὐτό σ. 1 Cor 7:5 v.l. In πρὶν ἢ συνελθεῖν αὐτούς Mt 1:18 domestic and marital relations are combined. (In marriage contracts in pap πρὸς γάμον τινὶ συνελθεῖν means ‘marry’. Also without πρὸς γάμον: BGU 970, 13 [II AD] συνηρχόμην τῷ προγεγραμμένῳ μου ἀνδρί).—M-M. TW.
