Levels of Znus according to R' Saadia

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.

Is Nida Yehareg v'al Yaavor?

In Parshas Emor we read "venikdashti besoch bnei yisrael," the mitzvah of kiddush Hashem. See Yesodei Hatorah Ch 5. If one would be forced to be boel a nida would one be required to forfeit his life, or may he be boel her? Nida is a chiyuv kares but it is different that other chiyuvei kareis, because kiddushin are not tofsin in chiyuvei kares except nida.

Shulchan Aruch E"H 195 and Shach ibid. 9 sayone would be required to forfeit his life. This is also the opinion of the Ritva (in the name of the Ra'ah), Pesachim 25a s.v. Ki Asa Ravin.

However, the following authorities say one is not required to forfeit his life and can be boel the nida:
Shu"t Pnei Yehishua II 44 (based on Rashi in Sanhedrin perek Ben Sorer)
Tosfos Sanhedrin 37 s.v. Hatorah
Gemara Temurah 29b
Sefer HaChinuch 296 (yehareg v'al yaavor is only for arayos if there is no tefisas kiddushin)
Rambam Igeres Kiddush Hashem (he says "bishas hashmad livol nida," implying nida is only a problem bishas hashmad.

Some sources that discuss this:
Encyclopedia Talmudis Vol. 22 columns 72 and 83
Perach Mateh Ahron, Mada p. 26
Od Yisrael Yosef Bni Chai siman 22