While it is always critical to examine primary sources, references such as BDAG (cited in this post) can make short work of identifying the semantic domain of words. Glosses in this lexicon are backed up by examples from the time, identify the way prepositions are used, etc. Ooooh that it were available to consult online. I have an online version that works in conjunction with Logos software but it is, of course also available as a codex.
Now, this word is an example where it is frequently misunderstood because of people using etymology rather than pondering extant usages. Often it gets you to the same place but just as often sends one into the weeds.
Etymology and cognates do often tell us things about the words when used in conjunction with a lexicon. I personally find etymology is often useful to me as a memory aid. Because there are closely related cognates I'm providing not only the entry for the word in question but several of the cognates that I find relevant and/or easily confused with the word we're considering in this question:
πορνεία, ας, ἡ (of various kinds of ‘unsanctioned sexual intercourse’: Demosth. et al.; LXX, En, Test12Patr; GrBar [in vice lists]; AscIs, Philo, apolog. exc. Ar. W. φθορά Iren. 1, 28, 1 [Harv. I 220, 14]) ① unlawful sexual intercourse, prostitution, unchastity, fornication, 1 Cor 5:1ab (CdeVos, NTS 44, ’98, 104–14); 6:13 (on 1 Cor 5–6 s. PTomson, Paul and the Jewish Law: CRINT III/1, ’90, 97–102); Hm 4, 1,
1. In a vice list (cp. AscIs 2:5) Ro 1:29 v.l. W. ἀκαθαρσία 2 Cor 12:21; Gal 5:19; Eph 5:3; Col 3:5. Differentiated fr. μοιχεία (Philo, Mos. 1, 300; s. also πορνεύω 1) Mt 15:19; Mk 7:21 (WGabriel, Was ist ‘porneia’ im Sprachgebr. Jesu?: Ethik 7, ’31, 106–9; 363–69); Hm 8:3; D 5:1 (the pl. denotes individual acts). On the other hand μοιχεία appears as πορνεία (cp. Sir 23:23) Hm 4, 1, 5. Of the sexual unfaithfulness of a married woman Mt 5:32; 19:9 (for the view that ref. is made in these pass. to forbidden degrees of marriage, s. 2 below.—JSickenberger, TQ 123, ’42, 189–206, ZNW 42, ’49, 202ff; KStaab [παρεκτός 2]; AAllgeier, Angelicum 20, ’43, 128–42. Cp. AFridrichsen, SEÅ 9, ’44, 54–58; AIsaksson, Marriage and Ministry in the New Temple, ’65, 127–42 [lit.]; s. also JFitzmyer, TS 37, 76, 197–226). Caused by lust D 3:3. διὰ τὰς πορνείας 1 Cor 7:2 (the pl. points out the various factors that may bring about sexual immorality; PTomson [s. above] 103–8). BMalina, Does Porneia Mean ‘Fornication’? NovT 14, ’72, 10–17. φεύγειν τὴν π. 6:18. Also ἀπέχεσθαι ἀπὸ τῆς π. 1 Th 4:3 (cp. Tobit 4:12). ἐκ π. γεννηθῆναι be an illegitimate child, a bastard (cp. Cephalion [II A.D.]: 93 Fgm. 5 p. 444, 5 Jac. ἐγέννησε ἐκ πορ.; Gen 38:24) J 8:41. On ἀπέχεσθαι τῆς πορνείας καὶ πνικτοῦ Ac 15:20 (cp. vs. 29; 21:25 and s. 2 below) s. the lit. s.v. πνικτός and in BBacon, The Apost. Decree against πορνεία: Exp. 8th ser., 7, 1914, 40–61. ② participation in prohibited degrees of marriage, fornication (s. Lev. 18:16–18; cp. Acts 15:20–29, s. Bruce, comm. Ac; 21:25) Mt 5:32; 19:9 (w. some favor RSmith, Matthew [Augsburg] ’89,100; RGundry, Matthew ’82, 91: “no need to adopt obscure definitions of πορνείας, such as marriage within the forbidden degrees. … The specific word for adultery does not appear in the exceptive phrase simply because a general expression occurs in Deuteronomy” [24:1], but s. BWitherington, NTS 31, ’85, 571–76: ‘except in the case of incest’. On these pass. s. 1.). ③ immorality of a transcendent nature, fornication, in imagery, of polytheistic cult in the mystic city Babylon, which appears in Rv as a prostitute with an international clientele. Fr. the time of Hosea the relationship betw. God and his people was regarded as a marriage bond. This usage was more easily understandable because some Semitic and Graeco-Roman cults were at times connected w. sexual debauchery (cp. Hos 6:10; Jer 3:2, 9; 4 Km 9:22; on the positive side, for concern about propriety on the part of some cults s. e.g. SIG 820 [83/84 A.D.], in which an Ephesian official assures Rome that the annual autumn fertility festival is conducted ‘with much chastity and due observance of established customs’. This level of conduct prob. stands up well against activities associated with celebration of a modern Mardi Gras.) Rv 19:2. μετανοῆσαι ἐκ τῆς π. αὐτῆς repent of her immorality 2:21; cp. 9:21. ὁ οἶνος τοῦ θυμοῦ τῆς π. the wine of her passionate immorality 14:8; 18:3 (on these passages s. θυμός 1 and 2). ὁ οἶνος τῆς π. 17:2. τὰ ἀκάθαρτα τῆς π. vs. 4 (ἀκάθαρτος 2).—V.l. for πονηρίας Hv 1, 1, 8 (Leutzsch, Hermas 447 n. 53). S. next entry 2.—DELG s.v. πέρνημι. M-M. EDNT. πορνεύω fut. πορνεύσω; 1 aor. ἐπόρνευσα; pf. 3 pl. πεπορνεύκασιν Ezk 16:34 (Hdt. et al.; LXX, En; TestAbr A 10 p. 88, 8 [Stone p. 24]; Test12Patr; Ar. 15, 4; Just.; Tat. 34, 3) of a variety of ‘unsanctioned sexual intercourse’. ① to engage in sexual immorality, engage in illicit sex, to fornicate, to whore, in Gk. lit. freq. in ref. to prostitution (s. L-S-J-M s.v.). In a gener. sense 1 Cor 10:8ab. Distinguished fr. μοιχεύειν ‘commit adultery’ (Did., Job 133, 22ff [quote fr. Hos 4:14], 25ff); D 2:2; B 19:4; Mk 10:19 v.l. Regarded as a sin against one’s own body 1 Cor 6:18. W. φαγεῖν εἰδωλόθυτα ‘eat meat offered to idols’ Rv 2:14, 20. ② engagement in polytheistic cult, fornication, in imagery (Phalaris, Ep. 121, 1), of polytheistic cult in the sense ‘practice image-worship/idolatry’ (πορνεία 3 and cp. Hos 9:1; Jer 3:6; Ezk 23:19; 1 Ch 5:25; Ps 72:27; En 8:2) Rv 17:2; 18:3, 9.—DELG s.v. πέρνημι. M-M. TW. πόρνη, ης, ἡ (cp. πέρνημι ‘export for sale’ [s. Schwyzer I 362] as of captive women exported for slavery Hom. et al.) (Alcaeus, Hipponax; Aristoph., and X., Mem. 1, 5, 4; PSI 352, 4 [254/253 B.C.]; POxy 528, 18 [II A.D.]; BGU 1024 VI, 4; LXX; PsSol 2:11; TestLevi 14:5, 6; Philo, Just.; Tat. 34, 2; loanw. in rabb.) ‘prostitute’. ① one engaged in sexual relations for hire, prostitute, whore lit. (since Alcaeus 109 + 110, 26 D.2 [115 Fgm. 3b 26 L-P.]) Lk 15:30 (cp. Pr 29:3; Test Levi 14:5 μετὰ πορνῶν); 1 Cor 6:15. Of Rahab (Josh 2:1; 6:17, 23, 25) Hb 11:31; Js 2:25; 1 Cl 12:1 (a πόρνη rewarded for a rescue also in Neanthes [200 B.C.]: 84 Fgm. 9 Jac.). W. tax-collectors as the lowest class of people, morally speaking Mt 21:31f. W. female flutists Ox 36. κολλᾶσθαι τῇ π. have to do with a prostitute (Sir 19:2) 1 Cor 6:16. ② a political entity hostile to God, prostitute, whore, fig. ext. of 1 (s. πορνεία and πορνεύω; Is 1:21; 23:15f; Jer 3:3; Ezk 16:30f, 35) as the designation of a government that is hostile to God and God’s people Rv 17:15f. ἡ πόρνη ἡ μεγάλη vs. 1; 19:2. Βαβυλὼν (q.v.) ἡ μεγάλη ἡ μήτηρ τῶν πορνῶν 17:5 (unless masc. πόρνων is to be read, s. next entry).—For the woman sitting on the beast cp. Cebes 5, 1, a beautifully adorned woman sitting on a throne. She is called Ἀπάτη, ἡ ἐν τῇ χειρὶ ἔχει ποτήριόν τι, from which she gives men to drink (ποτίζει Cebes 5, 2 as Rv 14:8), in order to lead them astray (πλανάω as Rv 18:23).—B. 1368. RAC III 1149–1212. DELG s.v. πέρνημι. M-M. EDNT. TW. p 855 πόρνος, ου, ὁ (cp. πόρνη; Aristoph., X.+ in the sense ‘male whore/prostitute’ and related terms; Sir 23:17; Philo, Leg. All. 8; Just.; Tat. 34, 3) ① one who practices sexual immorality, fornicator 1 Cor 5:9, 11; Hb 12:16; Rv 17:5, if accented as masc. (in N.27 as fem.; s. app. in Tdf. and early edd. of Nestle incl. 25th ed), but s. also 2. οἱ π. τοῦ κόσμου τούτου the (sexually) immoral persons in this world 1 Cor 5:10. W. other sinners Eph 5:5; 1 Ti 1:10; Rv 21:8; 22:15. Differentiated fr. an adulterer 1 Cor 6:9; Hb 13:4. Excluded fr. the Reign of God, w. others guilty of grave sins 1 Cor 6:9 (=Pol 5:3). ② a political entity hostile to God, fornicator, fig. ext. of 1: ἡ μητὴρ τῶν πόρνων mother of fornicators Rv 17:5 v.l., s.
1.—RBorger TRu 52, ’87, 48–50.—DELG s.v. πέρνημι. M-M. EDNT. TW.
Arndt, W., Danker, F. W., & Bauer, W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., pp. 854–855). Chicago: University of Chicago Press.
As the lexicon glosses and examples show for the word in question and its cognates the word was not limited to any particular kind of sexual [mis]conduct.
Getting back to the OP, what isn't clear to me is whether casual sex among consenting adults where the woman does not belong to someone already is being forbidden. The Torah, in Exodus 20:14 says not to commit "adultery". In the LXX that word is μοιχεύω G3431 not πορνεία. The two words appear in a list of vices together:
Mat 15:19 For out of the heart proceed evil thoughts, murders,
adulteries, fornications, thefts, false witness, blasphemies:
So this suggests that the Torah limited its prohibition to violating a husband's exclusive rights over a woman. She belongs to a man already and that man's rights must be respected.
Conversely, not much is made of prostitution in the Torah, though one is not to suffer their own daughter to be one:
Jdg 16:1 Then went Samson to Gaza, and saw there an harlot, and went
in unto her.
Lev_19:29 Do not prostitute thy daughter, to cause her to be a whore;
lest the land fall to whoredom, and the land become full of
When it comes time to marry though they want a virgin:
Lev_21:14 A widow, or a divorced woman, or profane, or an harlot,
these shall he not take: but he shall take a virgin of his own people
Deu 22:15 "Then the father of the young lady, along with her mother,
is to bring evidence of the young lady's virginity to the elders at
the gate. Deu 22:16 The father of the young lady is to then say to
the elders: 'I have given my daughter to this man as a wife, but he
despises her. Deu 22:17 Now look, he has invented charges against
her by saying, "I haven't found your daughter to be a virgin." But
here is the proof of my daughter's virginity.' Then they are to spread
the cloth before the elders of the city. Deu 22:18 "The elders of
that city will then take the man, punish him, Deu 22:19 fine him 100
shekels of silver, and then give them to the young lady's father,
because he had defamed a virgin of Israel. She is to remain his wife
and he can't divorce her as long as he lives. Deu 22:20 But if this
charge is true, and the evidence of the young lady's virginity weren't
found, Deu 22:21 they are to bring her to the door of her father's
house. Then the men of the city shall stone her with boulders until
she dies for doing a detestable thing in Israel—acting like a
prostitute while in her father's house. By doing this, you will remove
this evil from among you.
There is no such test for men.
Deuteronomy contains many acts considered deviant and forbidden but never simply says straight copulation is forbidden. In fact, King Solomon is the subject of an adoring paean celebrating his sexual prowess with an unmarried woman.
Polygamy is completely condoned by Moses and by Paul.
Paul upholds Torah but does he say that premarital sex is forbidden? It may be inferred that Paul believed that people became "one flesh" through the sex act:
1Co 6:15 You know that your bodies belong to the Messiah, don't you?
Should I take what belongs to the Messiah and unite them with a
prostitute? Certainly not! 1Co 6:16 You know that the person who
unites himself with a prostitute becomes one body with her, don't you?
For it is said, "The two will become one flesh."
So if that is the case then to copulate is to marry. In his view then it appears that "legitimate sex outside of marriage" is a bit of a non-thing.
I just found this which may suggest that Paul would not have considered copulation to effect marriage: https://judaism.stackexchange.com/questions/17039/does-casual-sex-establish-a-marriage