Okay, so what we’ve established is…
Gentiles are considered married anytime a man and woman are living together, regardless of any formal marriage certification under gentile laws.
Gentiles are considered divorced from each other whenever one of the two decides to leave the others home.
The above means that a gf/bf (that aren’t married under the law) would be considered married under the Sheva Mitzvos.
Violation of the marriage carries capital punishment.
Question: Regarding the last point, does violating the marriage mean, like by us Yidden, that the violation occurred with a married woman? IOW, a married woman with any male not her husband means both of them are guilty. But a married man with an unmarried woman does not constitute guilt. Or by gentiles does even a married man with an unmarried woman constitute guilt?
My guess (without having looked it up) is that only the former constitutes guilt. (IOW, similar to how it works, in this regard, by Yidden.)