ibn Ezra Shemos 20:14
Rav Saadia Gaon said that znus has many levels.
The most lenient is with an unamrried woman or widow,
and the level above that is a man with his wife while she is a nida, for after a few days she will be permitted to him.
Above that is with a married woman; if her husband would die she would be permitted to him.
Above that is a non-Jew who is not [a follower of] the Torah...If she would convert, ti would be permitted.
Above that is mishkav zachar which is never permitted.
Above that is extra-species, like man with a beast.