Why did Avraham ask Eliezer to swear in the way he did?

Question

Avraham asks his servant Eliezer to place his hand under his thigh to swear an oath regarding finding a daughter for his son. Chazal explain that he asked him to hold his (Avraham's) milah (male member) while swearing because it was a mitzvah item (similar to how in a rabbinic court one would need to hold a sefer torah) and that mitzvah was beloved to Avraham.

But was this really appropriate from a tzniut perspective? Moreover couldn't Eliezer have held the stone which Avraham used to perform the bris milah instead? Furthermore there is no indication in the text that the mitzvah of milah was more beloved to Avraham than, say, the mitzvah of welcoming guest. Why not instead have Eliezer swear while holding onto the entrance to the tent?

In short, please explain the necessity of the unusual process of this oath beyond the basic explanation given by the commentators.

Answer 1

I'd also read "place your hand beneath my loin" as "you must put the needs of my progeny before your own interests." (As in "yad hekdesh al ha'elyona")

Answer 2

From here:

This explains a curious detail of Abraham's behavior related by the Torah. When Abraham wanted his servant, Eliezer, to take an oath, he told him to "place your hand under my thigh"(Genesis 24:2). An oath is taken while holding a sacred object such as a Torah scroll or tefillin; here Abraham is telling Eliezer to swear on the part of his own body sanctified by the mitzvah of circumcision. Yet our sages tell us that "Abraham observed the entire Torah" though it was yet to be given [at Sinai]; so Abraham studied Torah, put on tefillin, affixed a mezuzah on his doorpost, etc. It would therefore seem that he had no shortage of "sacred objects" available to him. Why, then, did he have Eliezer place his hand "under his thigh," contrary to all common standards of modesty and propriety?

But as explained above, the import of Abraham's pre-Sinai mitzvot were of a wholly spiritual nature. Since G-d had not commanded him to do them, they remained human deeds, subject to the natural law that separated the spiritual from the material; while they had a profound effect on his own soul, the souls of his descendants, and the spiritual essence of creation, they had no impact on the material substance of the universe. The single exception was the mitzvah of circumcision, whose commandment by G-d constituted an empowerment to sanctify the physical. Thus, this was indeed the only sacred object available to Abraham.

Answer 3

Ibn Ezra explains that this was a standard form of displaying the servant's subservience to the master, and notes that it is still in practice in India:

והקרוב אלי שהיה משפט בימים ההם לשום אדם ידו תחת ירך מי שהוא ברשותו והטעם אם אתה ברשותי שים נא ידך תחת ירכי והאדון יושב וירכו על היד הטעם הנה ידי תחת רשותך לעשות רצונך וזה המשפט עודנו היום בארץ הודו

Many other commentators follow with some variation or another of this basic idea:

• Rashbam
• Bechor Shor
• Radak
• Chizkuni
• Rabbeinu Bechaye
• Tur
• Ibn Kaspi
• Ralbag

Answer 4

Tosfos in maseches Shavuos 38b s.v. hai daina asks, the gemara brings Rav Papa as saying that a person who is mashbiah with tefillin for the nekitas cheifetz is like someone who is to'eh b'd'var mishna and must be chozer, and we don't say that what's done is done. This is presumably because one should really be mashbiah using a sefer Torah, and tefillin don't suffice. Tos' question is that Avraham was mashbiah with his milah for the nekitas chefetz! How could he if a sefer Torah is required? The first answer given is that the gemara later says that for a talmid chacham tefillin are lechatchilah (צ"ע the dimyon between tefillin and the milah). The second answer given is that because in those days they didn't have any mitzvos greater than this (דעדיין לא נצטוו יותר), it fulfilled the same role that a sefer Torah does for us.

In short, because a d'var mitzvah must be held for the shvuah, and this was the best option.

I hope this was helpful.