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A prohibition stemming from sanctity is referring to secondary relationships forbidden by rabbinic law. And why is this called a prohibition stemming from sanctity? Abaye said: Whoever fulfills the words of the Sages is called sacred. Rava said to him: The language you use is not precise, since if so, whoever does not fulfill the words of the Sages is not called sacred, which implies that he is also not called wicked. However, anyone who transgresses the words of the Sages is in fact referred to as wicked.
Rather, Rava said that the reason why this is called a prohibition stemming from sanctity is that the term sanctity indicates differentiation or separation, and there is a principle קַדֵּשׁ עַצְמְךָ בַּמּוּתָּר לָךְ
you must sanctify yourself by refraining from that which is permitted to you by Torah law. The Sages decreed against secondary forbidden relations so that one would not eventually come to transgress Torah law.