Our Gemara on Amud Aleph discusses the scriptural supports for the rabbical prohibitions of sexual relations with certain relatives beyond the Biblical list, known as Sheniyos. The sheniyos are often one further generation from the original, for example one’s mother is a biblically prohibited sexual relation, while a grandmother is a sheniya.
The Gemara explains that each Sage offers a different support:
Rava says, For all these [severe] abominations have the men of the land done” (Leviticus 18:27) אֶת כׇּל הַתּוֹעֵבוֹת הָאֵל עָשׂוּ אַנְשֵׁי הָאָרֶץ״,
The word “These” in Hebrew takes an unusual form, and could also be understood to mean the harsh ones, i.e., the severe transgressions. This proves by inference that there are also lesser transgressions that are, as it were, soft ones. And what are those soft ones? They are secondary forbidden relationships.
Rav Yehuda said that the prohibition of secondary forbidden relationships is learned from here: As it says with regard to King Solomon: “He weighed [izzen] and sought out, and set in order many proverbs” (Ecclesiastes 12:9), and Ulla said that Rabbi Elazar said: Before Solomon came, the Torah was like a basket [kefifa] without handles [oznayim play on words izzen], until Solomon came and made handles for it. By means of his explanation and proverbs he enabled each person to understand and take hold of the Torah, fulfill its mitzvos, and distance himself from transgressions, like handles help one hold a vessel.
Rabbi Oshaya said that the prohibition of secondary forbidden relationships is learned from here: “Avoid it, pass not by it; turn from it, and pass on” (Proverbs 4:15). One must distance oneself from any prohibited act and not pass near a place of forbidden objects or situations.
Rav Kahana said that the prohibition of secondary forbidden relationships is alluded to from here, in the verse stated at the conclusion of the verses discussing the halakhos of forbidden relationships: “Therefore shall you protect [ushmartem] my guarded items [mishmarti], that you do not any of these abominable practices” (Leviticus 18:30). This means: Establish a safeguard [mishmeret] for My guarded possessions.
The Maharal (Be’er Hagolah 1:2) explains that each of these sources represent a different idea behind rabbinic ordinances.
Rava’s point is that since the Torah considers certain relatives severely forbidden, it is only logical that there should be a category that is mildly forbidden. And though the Torah did not enumerate them, we should refrain from them due to their proximity with more severe prohibitions, because we should infer by their very proximity that they are mildly prohibited.
Further, according to Rabbi Yehuda, the rabbinic enactments are like handles on a vessel. That is to say, they allow you to hold it properly without dropping it. The observance of these secondary prohibitions promote the observance and understanding of the primary prohibitions. This is different than Rava. Here, the secondary prohibition is a prevention. But according to Rava, the secondary prohibition is itself deemed prohibited by association, just to a lesser degree.
According to Rabbi Oshaya, the secondary prohibitions are not intrinsically forbidden in a milder form like Rava, nor a safeguard like Rav Yehuda. Instead they serve their own value by deliberately withdrawing into a permitted area, so as to stay far away from sin. It’s subtly distinct from Rav Yehuda. As Rav Yehuda still sees the sheniya as a handle, that is to say somehow still connected to the prohibition. While Rav Oshaya also sees it as a safeguard, but coming from the outside to stay far away from sin.
Finally, Rav Kahana sees the mitzvos, that is the Biblical prohibitions, as a protection in and of themselves, as the Torah is designed to promote physical and spiritual well-being. The rabbinic injunction is a protection on top of that protection.
Maharal likens this to parts of the body designed to protect the organism, such as the eye, which also has an eyelid, to protect the protector. The organ is not complete without the protection, even though it functions. Similarly, the Biblical prohibition is the protector, but the rabbinic law is the completion of the “organ”.
You might wonder if the Torah wanted to forbid something, it should simply do so. Why rely on the sages to add additional structures? But looking at the different ideas expressed above, it seems that rabbinic prohibitions are similar to not stepping past the yellow line on the subway. It doesn’t need to be a law; it’s just not safe or smart. However, later on, a different government may establish the safety guideline into a formal law.
For more on this, see upcoming Psychology of the Daf Yevamos 26
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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