This is correct, although the Rambam (Isurei Bi'ah 12:1) and Shulchan Aruch (EH 16:1) rule that it is Min ha'Torah. The Magid Mishnah says that this is because the Rambam rules like Rebbi Shimon, although the Vilna Gaon explains that the Rambam held that according to the Gemara in Kidushin 68b the Rabanan hold that Lo sis'Chatein refers to all gentiles.

However, having sexual relations with a non-Jew is forbidden Min ha'Torah if the act is known to ten Jews (see Rambam ibid. 12:4 and Mordechai Yevamos 4:108). If two people are married, we consider it as public knowledge that they are having marital relations and therefore such a marriage is forbidden Min ha'Torah (see Aruch ha'Shulchan EH 16:5).

Dov Freedman