Importance of describing the lack of Rivkah's intimate experience

Question

Gen 24:16:

"וְהַנַּעֲרָה טֹבַת מַרְאֶה מְאֹד בְּתוּלָה וְאִישׁ לֹא יְדָעָהּ"
The maiden was very beautiful, a virgin, whom no man had known.

1. We know (Rashi on Genesis 34:7) that the nations already strengthened their observance of Arayot after the flood.

2. The sexual misbehavior was only a norm in Canaan "וּכְמַעֲשֵׂה אֶרֶץ־כְּנַעַן אֲשֶׁר אֲנִי מֵבִיא אֶתְכֶם שָׁמָּה לֹא תַעֲשׂוּ וּבְחֻקֹּתֵיהֶם לֹא תֵלֵכוּ׃", I don't find this with Semitic nations.

3. Torah does not mention this fact about any other of our Foremothers or other Jewish women (oh, Avishag in this Parsha's Haftarah).

4. Rivkah was only 3yo, but Rachel and Leah were grown up, and there's no mentioning of their experience (they came from the same family).

Why this description is important?

Answer 1

I don't offer an answer to your overall question, but to answer your point 1:

We know (Rashi on Genesis 34:7) that the nations already strengthened their observance of Arayot after Noah and his daughters' accident.

it pays to look at both the local Rashi and the Rashi to which you refer. The Rashi to which you refer says:

וכן לא יעשה. לְעַנּוֹת אֶת הַבְּתוּלוֹת, שֶׁהָאֻמּוֹת גָּדְרוּ עַצְמָן מִן הָעֲרָיוֹת עַל יְדֵי הַמַּבּוּל (בראשית רבה):

That is, they refrained from rape (la'anot) of virgins, because of a general fencing off from "arayot". Thus, what Shechem did was out of bounds even by gentile standards, as a societal norm. That does not necessarily mean that unmarried young men and women did not engage in intercourse. (Is there a prohibition on a panuy with a penuya?

Then, look at the actual text of the local Rashi (on 24:16):

ואיש לא ידעה. שֶׁלֹּא כְּדַרְכָּהּ, לְפִי שֶׁבְּנוֹת הַגּוֹיִם הָיוּ מְשַׁמְּרוֹת מְקוֹם בְּתוּלֵיהֶן וּמַפְקִירוֹת עַצְמָן מִמָּקוֹם אַחֵר, הֵעִיד עַל זוֹ שֶׁנְּקִיָּה מִכֹּל:

The assumption is explicit in Rashi that the women were promiscuous in unnatural ways. Maybe you could argue that this is a dispute among midrashim, but consider that Rashi's source, in both cases, is Midrash Rabba.

Perhaps you could say that indeed, because of the increase in gentile society for sexual purity at marriage, this was why, when boys and girls were being boys and girls, the gentile girls engaged in 'unnatural acts' to preserve their maidenhead. This is what Rashi said, after all.

Answer 2

R. Naftali Tzvi Yehuda Berlin has an interesting explanation in his commentary to this verse, which can perhaps address your questions. He interprets the verse as saying not simply that Rivkah had never engaged in any sexual behavior, but that none of the shepherds even knew her (i.e. לא ידעה is taken very literally) because she had never come to the well before. When Avraham's servant saw that the shepherds knew all the other girls but not Rivkah, he understood that she had good parentage and/or that she was a tzenuah, and thus did not hang out with the men at the well, unlike the other girls. This, says R. Berlin, is one of the reasons the servant chose Rivkah.

והנערה טובת מראה מאד וגו' ביאר הכתוב שלשה טעמים מדוע פגע בה העבד יותר מבאחרות א' שהיא טובת מראה מאד יותר משארי בתולות שראה שם ב' ואיש לא ידעה שהתבונן בה שאין לה דבר עם אנשים כמו שראה שארי בתולות שהכל מכירין אותן באשר היו רגילות בכל יום להיות עם הרועים אצל הבאר וגם לא היו צנועות כל כך אבל אותה בתולה לא יצאה עד היום לשאוב על כן לא ידעה אדם ומזה התבונן העבד שהיא מיוחסת ומשום הכי לא היתה רגילה בכך או משום צניעות ג' ותרד העינה ותמלא כדה ותעל ראה שהיא לא שחה עצמה לשאוב ככל השואבות שזה אינו דרך צניעות כדאיתא בשבת לענין לקט אלא ירדה לתוך העין ותעל

So to answer your main question, it was important for the Torah to tell us this because this was one of the ways that the servant was able to pick her as the right girl.

To answer your specific questions, the first two are answered once we establish that the Torah was not simply talking about actual sexual misbehavior, and the last two can be answered by saying that this description simply wasn't true of the others. In fact it is explicit in Genesis 29:6 that the shepherds did know Rachel.