Exodus 20 commands the people to not תִנְאָף, translated "commit adultery." What would have been included in their concept of this? Would a man having sex with another man have been considered as תִנְאָף?
In regards Tin'af (תִּֿנְאָֽ֑ף) "Adultery" in Leviticus 18:20, Sefer HaChinukh 35:1 reads:
To not reveal the nakedness of a man's wife: To not have intercourse with a man's wife, as it is stated (Exodus 20:13), "You shall not commit adultery." And the explanation comes that the undifferentiated expression, "adultery," indicates with a man's wife, as they, may their memory be blessed, said (Rashi on Exodus 20:13), "Adultery is only with a man's wife." And this negative commandment is repeated in the Order of Achrei Mot, as there it is written explicitly (Leviticus 18:20), "And to your neighbor's wife, etc."
In regards to Leviticus 18:22, the phrase "וְאֶ֨ת־זָכָ֔ר לֹ֥א תִשְׁכַּ֖ב" : "V'Et Zakar Lo-Tishkav" would never refer to Adultery. [See Yayikra 18:22] * "Adultery" / Tin'af (תִּֿנְאָֽ֑ף) spoken of in [Shemot 20:13] is defined in Shemot 20:14 : "לֹֽא־תַחְמֹ֞ד אֵ֣שֶׁת רֵעֶ֗ךָ" : you shall not covet your neighbor’s wife.
Homosexual sex is dealt with earlier separately in Leviticus 18:22
Do not have sexual relations with a man as one does with a woman; that is detestable.
"You shall not commit adultery.
Verb - Qal - Imperfect - third person masculine singular
Strong's Hebrew 5003: To commit adultery, to apostatize
usually of man, always with wife of another
Verse 14. - Thou shalt not commit adultery. Our second duty towards our neighbour is to respect the bond on which the family is based, and that conjugal honour which to the true man is dearer than life. Marriage, according to the original institution, made the husband and wife "one flesh" (Genesis 2:24);
Jesus confirms this in Matthew 5:28
But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
Would the ancient Hebrew word/concept of adultery (תִנְאָף) have included homosexual sex?
Not according to Jesus.