1 Corinthians 6:9

μὴ πλανᾶσθε· οὔτε πόρνοι οὔτε εἰδωλολάτραι οὔτε μοιχοὶ οὔτε μαλακοὶ οὔτε ἀρσενοκοῖται οὔτε κλέπται οὔτε πλεονέκται,^[1] οὐ μέθυσοι, οὐ λοίδοροι, οὐχ ἅρπαγες βασιλείαν θεοῦ κληρονομήσουσιν.

Do not be deceived! Fornicators, idolaters, adulterers, male prostitutes, sodomites [ἀρσενοκοῖται], thieves, the greedy, drunkards, revilers, robbers—none of these will inherit the kingdom of God.

1 Corinthians 6:9–10 (NRSV)

I'm a classicist and I'd never heard the word ἀρσενοκοίτης (despite having read some Aristophanes, which will usually get you pretty far in the sexual vocabulary realm), so when its precise meaning became a point of contention in an argument an acquaintance was having on the internet, I decided to do what I would for any ancient Greek word of uncertain meaning: I looked it up in the TLG, which is basically a database of as much premodern Greek text as a team of classicists can digitize in fifty years.

A few notes based on that corpus search:

1. As far as I can tell, the TLG records no usage prior to Paul of either ἀρσενοκοίτης or its alternative spelling, ἀρρενοκοίτης. (There is a use in the Anthologia Graeca which is by its nature of mixed dates, but it seems to refer to a 9th century Byzantine emperor, so I'm fairly confident in giving it a post-Pauline date.)
2. It's rare-rare: even with the two spellings, it's only recorded 59 times in my (admittedly out of date) instance of the TLG, and a good number of those seem to be just people directly quoting Paul.

Whether Paul created the word, or whether he is simply using a word that was previously used in no-longer extant texts or speech is unclear. Either way, there's still a sense of backwards-projection to our understanding of it: we base our guess of what Paul meant on those who came after him and interpreted him, rather than the usages that he would have been familiar with (if, again, he didn't simply coin the phrase).

On first look through the uses, the thing that caught my eye was a use in the Scholium in Aristophanem, or "Commentary on Aristophanes". This was basically the only secular context of ἀρσενοκοίτης I could find. In one note the scholiast says of line 153 of the Wealth: ἐνταῦθα διασύρει τὴν τῶν Ἀθηναίων διαγωγὴν, ὅτι ἦσαν ἀρσενοκοῖται καὶ φαῦλοι ("Here, he [Aristophanes] ridicules the passtime of the Athenians, that they are [ἀρσενοκοῖται] and common"). This was interesting to me because it's definitional, in that the scholiast is classing some manner of behavior as ἀρσενοκοίτης.

This, for the record, is the behavior:

CHREMYLUS: And what of the Corinthian courtesans [ἑταίρας]? If a poor man offers them proposals, they do not listen; but if it be a rich one, instantly they offer their buttocks for his pleasure.

CARIO: 'Tis the same with the lads [παῖδάς]; they care not for love, to them money means everything.

(Plut. 149–54, translator anonymous but presumed to be Oscar Wilde)

Note that the sex work is done by the Corinthians; the scholiast interprets these lines as making fun of the purchase of sex, then, by the Athenian speakers and Athenians in general. Also note that, though male (ἄρσην) appears in the compound, and the speakers are both male, in these lines the sex workers are explicitly female (ἑταίρας) and only implicitly male (παῖδάς can refer to children of any gender,^[2] but here the context does seem to suggest male children), so in usage it might not be so cut-and-dried.

There's a lot of ways that the sense of the word might not be exactly what you first think when you break it down to its component parts—etymology isn't definition. Even in terms of strictly one-to-one interpretations of compounds, there are other senses to the words in the compound: ἄρσην can mean male, but it can also mean "coarse, tough", and κεῖμαι ("to lie", the ο-grade root of which makes its way into ἀρσενοκοίτης)^[3] has a vast swath of more-or-less metaphorical meanings, from "to lie dead" to "to sleep".

Anyway, the point of all this is that even on the basis of a pretty cursory look, the sense of this word is nowhere near as firmly established in the corpus as a dictionary entry might imply. I'm not saying definitively that it doesn't mean "homosexual"; it might! But it might not. And if "homosexual" was all Paul wanted to say, he had a well established set of words to do so, and he chose not to use them. It's worth taking a minute to think about why.