These verses are mistranslated in pretty much every English Bible commonly available. Both verses are printed below in Greek, then transliterated, and correctly translated, with explanations about the translations.
1 Corinthians 6:9
Ἤ οὐκ οἴδατε ὅτι ἄδικοι Θεοῦ βασιλείαν οὐ κληρονομήσουσιν; Μὴ πλανᾶσθε· οὔτε πόρνοι οὔτε εἰδωλολάτραι οὔτε μοιχοὶ οὔτε μαλακοὶ οὔτε ἀρσενοκοῖται…
I ouk idhate oti adhiki Theou vasilian ou klironomisousin? Mi planasthe; oute porni oute idhololatrai oute mikhi oute malaki oute arsenokitai…
(Transliteration of Modern Greek pronunciation.)
Or haven’t you known that the unjust will not inherit the kingdom of God? Do not be misled; neither fornicators nor idolaters nor adulterers nor soft ones* nor those who lie with males…**
1 Timothy 1:10
…πόρωοις, ἀρσενοκοίταις, ἀνδραποδισταῖς, ψεύσταις, ἐπιόρκοις, καὶ εἴ τι ἕτερον τῇ ὑγιαινούσῃ διδασκαλίᾳ ἀντίκειται,
…pornis, arsenokitais, andhrapodhistais, psevstais, epiorkis, kei i ti eterov ti iyi-einousi dhidhaskalia antikitai,
(Transliteration of Modern Greek pronunciation.)
…to fornicators, to those who lie with males,** to kidnappers, to liars, to perjurers, and if any other thing opposes healthful teaching,
*Soft ones: The Greek word μαλακοὶ (mala-KEE) is a plural noun, derived from the adjective μαλακός (mala-KOS). The adjective means infirm (from illness), or soft or fine, but in meaning soft or fine, is restricted to describing material or clothing. It describes the type of clothing worn by very wealthy people. This adjective was used in Luke 7:25, when Jesus asked the crowd if they had gone out to the wilderness expecting to see someone dressed in fine clothing. A noun derived from this adjective was most often used to mean people who are ill. Alternately, it would seem to suggest the type of people who wore soft or fine clothing, which, in the first century, would be the very rich. Jesus Himself indicated how difficult it would be for wealthy people to enter the kingdom. (Luke 18:24)
**Those who lie with males: The Greek word ἀρσενοκοῖται (arseno-KEE-tay) (the form used in 1 Timothy is ἀρσενοκοίταις [arseno-KEE-tays]), is formed by combining a form of the noun αρσην arsin, which means male, with the construction κοιτ kit-, a derivative of the verb κειμαι kimei, which means lie down. Combined, the word refers to people who lie down with males. What remains to be determined is whether the word is referring to males lying with males, or females lying with males. Ordinarily, to determine if a Greek noun is masculine or feminine, one looks at it in the nominative case with the definite article. For example, ὅ ἀδελφός – o adhelfos, the brother, is in the nominative case, and both the ός ending and the definite article ὅ tell us the noun is masculine. But the word used in these two verses presents a small challenge to us, because in first century literature, it never appears with a definite article. (In fact, outside these two passages, it never appears at all!) Of course, we could simply look it up in a modern Greek dictionary, and it would tell us the word is masculine and means homosexual. But is that the end of it? Actually, no. The dictionary’s definition and assignment of gender are based on a few centuries of preconceived notions about what Paul was saying, and not on actual usage from the first century. So in this case, the dictionary can’t answer the question for us. We need to look back to the word itself, and its context, to search for clues.
The last two letters of the word in 1 Corinthians, and the last three in 1 Timothy, are where we need to look first. Greek nouns are declined according to case. That is, the ending of a noun changes to indicate how the word is being used in the sentence. We have something similar with English pronouns: We use the word I as a subject, but me as an object. For all intents and purposes, I and me mean the same thing. But it is incorrect to say Me want a book, or Give I a book. In 1 Corinthians, the word is in the nominative case, and the ending is one that is often feminine. This would suggest that the word is referring to women lying with males. In 1 Timothy, the word is in the dative case, which in English corresponds to putting the word to before the noun. (Example: Give the book TO ME.) And again, the ending is one that is often feminine.
The fact that this word is not found in any literature prior to the first century, and then only in Paul’s two uses, suggests that Paul himself coined it. Although there was no such word as homosexual at the time, there were expressions in common use to indicate sexual activity between persons of the same sex. Had Paul intended to refer to homosexuals here, common sense would have him use expressions people already knew and understood. But he never used any of those expressions in his writing. So the creation of a new word suggests a different concept. In addition, had he intended for his new word to be understood to refer to males, he would have given it a different ending: The plural ending οι (οις in the dative case) is never feminine and would have served just that purpose. But he didn’t do that, either. So even without a definite article to prove the point, the evidence so far suggests that Paul was speaking about women when he used this word, not men.
There is more evidence to consider: First, when properly translated, Scripture contains no prior condemnation of homosexuality, and the Hebrew Old Testament contains the record of two same-sex marriages, neither condemned by God. Paul, as a Jewish scholar, could not have been ignorant of this. So for him to suddenly, and without precedent, introduce a condemnation of homosexuality, without a word of explanation, would make absolutely no sense, and would probably have created an uproar in the early churches. Church history documents that same-sex marriages existed, and continued, in the Christian churches up until around the 13th or 14th century.1
This word was used only twice in the first century, both times by Paul, and was not used again until the second century. In the second century, writers used it to mean female prostitutes.2 This raises the question of why Paul would create a new word for prostitutes. A valid reason for doing so was that he had already used the original word for prostitutes to mean something else. Centuries before, in earliest Greek, the πορν– (porn-) root referred only to prostitutes and prostitution. But over the centuries, it had expanded in meaning, and now referred to any extra-marital sex. Paul had already used the word πόρνοι to mean fornicators. Since prostitution was both a business and a form of worship of the fertility goddess, some could have argued that Paul didn’t have prostitutes in mind when he said fornicators. So the word ἀρσενοκοῖται covers that loophole. In later centuries, at least one writer understood the word to mean something a husband did with his wife!3 Historically, only relatively recently have people begun to think this word referred to homosexuals, and newer translations actually render the word that way, despite the lack of grammatical or historical support.
Is it true that Paul coined ἀρσενοκοῖται from Lev. 20:13 in the Septuagint?
There is a misconception currently held by some Christians that Paul coined the word ἀρσενοκοῖται from Lev. 20:13 as found in the Greek Septuagint (LXX), which is the oldest translation of the Hebrew Bible.:
Και ος αν κοιμηθη μετα αρσενος κοιτην γυναικος βδελυγμα εποιησαν αμφοτεροι θανατουσθωσαν ενοχοι εισιν·
The idea is based upon the existence of the words αρσενος κοιτην in that verse, but this is flawed scholarship. Since αρσενος means male, and κοιτην means bed, ANY Greek sentence that mentions a male and a bed will have forms of those two words in it. Leviticus 18:22 and 20:13 are not the only verses in the Septuagint containing those words, as seen below.
Και νυν αποκτεινατε παν αρσενικον εν παση τη απαρτια και πασαν γυναικα ητις εγνωκεν κοιτην αρσενος ζωγρησατε αυτας
Πασαν την απαρτιαν των γυναικων ητις ουκ οιδεν κοιτην αρσενος ζωγρησατε αυτας
And now kill every male among all children. But every woman who has not known the bed of a male, take them alive. All the women children who have not gone to the bed of a male, take them alive.
Και ουτος ο λογος ον ποιησετε παν αρσενικον και πασαν γυναικα γινωσκουσαν κοιτην αρσενος αναθεματιετε
Και ευρον απο των κατοικουντων ιαβις γαλααδ τετρακοσιας νεανιδας παρθενους αι ουκ εγνωσαν ανδρα εις κοιτην αρσενος και ηγον αυτας εις την παρεμβολης εις σηλω η εστιν εν γη χανααν
And this is the word that you will do: every male, and every woman who has known the bed of a male, you will destroy. And they found among the inhabitants of Jabesh-Gilead four hundred young virgins who had not known a man in the bed of a male, and the brought them into the camp into Shiloh which is in the land of Canaan.
In each of these four verses, the phrase “bed of a male” is in relation to women who have not known that location, that is, women who were virgins. In Leviticus, however, we have a different set up. Lev. 20:13 includes the phrase μετα αρσενος κοιτην γυναικος. In this verse, αρσενος (a male) is preceded by μετα (with), while κοιτην (a bed) is paired with the genitive γυναικος (of a woman). This agrees exactly with the Hebrew text, that is, with a male (in) a woman’s bed. Ἀρσενοκοῖται, on the other hand, is NOT derived from the word for bed, but from the verb meaning “lie down.” This verb, κειμαι, in some of its forms, uses the construction κοιτ-. Therefore, ἀρσενοκοῖται does not mean male beds, but rather, those who lie with males.
1“Same-Sex Unions in Premodern Europe” by John Boswell.
2 Sibylline Oracles 2.70-77, Acts of John, Theophilus of Antioch’s Ad Autolycum
3 John the Faster, Patriarch of Constantinople, “Penitential.” circa AD 575.