God gave all mankind -- including the Gentiles -- a conscience -- which was the Ten Commandments "written in their hearts" (Rom 2:14,15). So yes, the Gentiles did follow their conscience, and adultery was unacceptable among all Gentiles.
14 for when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, 15 who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them) 16 in the day when God will judge the secrets of men by Jesus Christ, according to my gospel.
Romans 2:14-15
This is describing
New Covenant Gentiles, not Gentiles before the law of Moses.
The law written on their hearts is New Covenant language.
31 "Behold, the days are coming, says the Lord, when I will make a new covenant with the house of Israel and with the house of Judah-- 32 not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the Lord. 33 But this is the covenant that I will make with the house of Israel after those days, says the Lord:
I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people.
Jeremiah 31:31-33
The Ten Commandments were God's Kingdom laws before mankind was even on the earth.
However it was the New Covenant that He began to "write them on the hearts of men.
Paul is comparing New Covenant Gentiles who don't have the law with Jews who do have the law.
...and circumcision is that of the heart, in the Spirit, not in the letter;
17 Indeed you are called a Jew, and rest on the law, and make your boast in God, 18 and know His will, and approve the things that are excellent, being instructed out of the law, 19 and are confident that you yourself are a guide to the blind, a light to those who are in darkness, 20 an instructor of the foolish, a teacher of babes, having the form of knowledge and truth in the law. 21 You, therefore, who teach another, do you not teach yourself? You who preach that a man should not steal, do you steal? 22 You who say, "Do not commit adultery," do you commit adultery? You who abhor idols, do you rob temples? 23 You who make your boast in the law, do you dishonor God through breaking the law? 24 For "the name of God is blasphemed among the Gentiles because of you," as it is written. 25 For circumcision is indeed profitable if you keep the law; but if you are a breaker of the law, your circumcision has become uncircumcision. 26 Therefore, if an uncircumcised man keeps the righteous requirements of the law, will not his uncircumcision be counted as circumcision? 27 And will not the physically uncircumcised, if he fulfills the law, judge you who, even with your written code and circumcision, are a transgressor of the law? 28 For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; 29 but he is a Jew who is one inwardly;
and circumcision is that of the heart, in the Spirit, not in the letter; whose praise is not from men but from God.
Romans 2:17-28
JLB