Why is "marriage effective contingent upon the approval of the Sages"?

Question

In Gittin 33a, Kesuvos 3a we learn that Rabbanan have the power to uproot a kiddushin because:

כל דמקדש אדעתא דרבנן מקדש ואפקעינהו רבנן לקידושין מיניה

When a man betroths a woman, he does so under the conditions laid down by the Rabbis, and in this case the Rabbis annul his betrothal.

My question revolves on how to fully understand this principle:

Source for it? Is it d'oraisa or d'rabanan?
Why does this principle exist by kiddushin/ geirushin specifically?
What is the rationale for it existing? We don't extend this principle in other areas like "when a person makes a sale, he does so under the conditions laid down by the Rabbis"

A peculiar remark about kiddushe kesef, Because the rule of hefker Bet din hefker. This mains that the Bet din has a power to neutralize the monetary ownship. So the Money of kiddushin may be confiscated from the husband and there is no kiddushin. — kouty, Jan 04 '19 at 06:18
I have no time to answer but see Rashi who comments two pshatim. The pshat he doesn't reject is following a reasoning. People act following the custome, that is teached by our rabanim. And the rabanim fixed many problems by anticipation. — kouty, Jan 04 '19 at 13:35
@kouty Your first comment seems to be in line with the Gemara’s statement of “that’s fine by Kiddushei Kesef,” but what about Kiddushei Bi’ah? What right do Chazal have to make it Bi’as Zenus? — DonielF, Jan 04 '19 at 14:02
@kouty where can I find that Rashi? By Gittin 33a or somewhere else? — alicht, Jan 04 '19 at 14:08
Gittin 33a. Of course. There is a easy and interesting to at the same place — kouty, Jan 04 '19 at 14:26
@Doniel yes, the Rashi explains 2 pshatim in עשו רבנן את בעילתו בעילת זנות — kouty, Jan 04 '19 at 14:28
Possible duplicate of Why is Kiddushin Performed "according to the tradition of Moshe and Israel"? — user15464, Mar 27 '19 at 23:29
@user15464 the focus of these questions seems very different. That one is about phraseology, this one is about source/rationale — Y e z, Mar 28 '19 at 02:48

score 1 · Answer 1 · answered Jul 31 '20 at 16:28

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When a man marries a woman he says הרי את מקודשת לי כדת משה וישראל, "You are married to me according to the law of Moses and Israel." In other words, he only wants the marriage to go into effect if it is consistent with the laws of the Torah (דת משה) and other rabbinic decrees (דת ישראל).

answered Jul 31 '20 at 16:28

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Are you suggesting everyone always uses kiddushin al tnai? – Double AA Jul 31 '20 at 16:33
Doesn’t Tosafos imply that it’s the reverse? – Alex Jul 31 '20 at 18:02
@DoubleAA, yes everyone is mikadesh al tna (see my next comment) – Rafael Dec 27 '20 at 02:48
@Alex, Tosfos explains that b/c bes din has control over everyones money, they can force us to be mikadesh al tnai. – Rafael Dec 27 '20 at 02:48

Why is "marriage effective contingent upon the approval of the Sages"?

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