I would agree that it's not answered as clearly as some (I doubt all) might prefer.
(But I recall Bill Klinton saying he knew that getting a bit of head in the oval orifice
wasn't...)
There ARE examples of
'prohibited relationships' which seem to be of a sexual nature, such as incest, or "a man lying with a man as with a woman." They do not seem to be called "one flesh" (I might even be tempted to consider that they are not POSSIBLE to be such) - but are at least arguably related.
The mind boggles. Is a boy getting fellated by his sister 'incest'? Or should we suggest that, too, is 'off limits'? As for what two rainbow-hued men might do that is prohibited, I tend to think, personally, that ANYTHING that might be gratifying in the 'marriage bed' (which is "undefiled") is probably off-limits.
But the key probably lies in the Hebrew word 'erva', used repeatedly in the incest (et al) verses, as well as the famous-here Lev. 18:18, which probably does NOT utterly prohibit marriage to a sister in her lifetime, but does say that to "expose her nakedness," using that same word עֶרְוָה or
erva, is whatever shouldn't be done.
BTW, the famous "first use" of that word is the story of Noach, and his son Ham, which resulted in the cursing of Canaan. There is a LOT of midrash (and even outright argument
) over what Ham actually did, but it was clearly prohibited, even before Moses Wrote it down.
The word is also rendered as having to do with "pudenda," or "genitalia," and that certainly seems to be a bummer for a lot of Samuel's sex-ed kids.
Bottom line: I dunno. But at least I see why some suggest to "err on the side of...caution."