As I understand it, the halacha interprets "lo tina'af" as prohibiting sex between a man (married or unmarried) with a married woman not his wife. Is a gentile woman, in a marriage between gentiles, ...
We know that if a man has relations with a non-Jewess, the halacha is A zealot may kill him in the act (Sanhedrin 82a). Both R' Schneersohn (the last one -- in Sichos) and R' Tzadok haCohen miLublin ...
If a Jewess married to a Jew has a child from a non-Jew is that child a mamzer? Is there agreement amongst the poskim about this?