According to the gemarah (Sanhedrin 21 I think) milk was considered the limb of a living being - eiver min ha'chai - until matan torah. If so, how could Avraham serve it to his guests if a ben Noach was forbidden to consume it? Is this not placing a stumbling block before a blind man?
Quoting from here, #7:
There is a general prohibition of "eating a limb from a live animal" (ever min hachai), which logically should also include milk, the product of a live animal. Ever min hachai is actually one of the Seven Noahide Laws which the Jews observed prior to Sinai (and which has applied to all humanity since the days of Noah).
However, upon receiving the Torah, which refers to the Land of Israel as "flowing with milk and honey" (Exodus 3:18), dairy products became permitted to the Jews. In other words, at the same moment that their meat became prohibited, dairy became permitted. They ate dairy on that original Shavuot, and we do today, too.
If the Jews ate dairy for the first time at Mount Sinai, this raises the question how Abraham could have fed dairy products to his three guests (Genesis 18:8).
The answer requires a technical understanding of the prohibition of ever min hachai, "limb from a live animal." One way is to define a "limb" as a piece of meat which contains bones and/or sinews. It is this type of ever min hachai which has always been forbidden to non-Jews. This prohibition does not include milk, because although milk comes from a live animal, it does not contains bones or sinews. Hence, Abraham was permitted to feed milk to his non-Jewish guests.
There is a second, expanded definition of ever min hachai, which encompasses all products from a live animal -- including milk. It is this definition which is prohibited to Jews. Thus it was not until the giving of the Torah, with its reference to "land of milk and honey," that dairy products became permitted to Jews.
This distinction is spelled out clearly by the great Rabbi Shlomo Kluger, in "HaElef Lecha Shlomo" (Yoreh Deah 322).
Sanhedrin 21 has the halachos of a king. I did not see a reference to matan torah and milk in it. I checked other sources and the reference is to Bechoros 6b (ArtScroll 6b2 notes 24, 25, and 27) Part of the reasoning as to why Avraham could serve milk is based on an implication that milk was not considered eiver min hachai (for Ben Noach) that seems to be derived from those notes.
Additionally, the fact that
7 And Abraham ran unto the herd, and fetched a calf tender and good, and gave it unto the servant; and he hastened to dress it. 8 And he took curd, and milk, and the calf which he had dressed, and set it before them; and he stood by them under the tree, and they did eat.
Note there are two commentaries here: First he serves milk and then later meat while the other commentary says at this point Avraham is a Ben Noach and not bound by the prohibition s of waiting between milk and meat and the reverse. For a Ben Noach can even eat together milk and meat together and therefore no conflict within.
Note that the implication here is that even eating milk and meat together is permitted a Ben Noach. Thus milk by itself is of course permitted.
Milk is blood changed to feeding material which in its turn is changed to blood.
Thus, he shows that it is not forbidden to ben Noach as aiver min hachai.
Also note the explicit prohibition of consuming blood. That is because it is not aiver min hachai. That is, while it is contained within the body of the animal, it is not a dependant limb of that animal (like a leg).
In his Sefer Moadim U’Zmanim(V.8 - 79:319), Rav Moshe Sternbuch brings a novel reason for this Minhag. He says it is based on the Gemarah in Bechoros(6B) that discusses whether milk taken from a cow was considered Eiver Min HaChai (eating the limb of a live animal) before Matan Torah. If that is so then it is at MatanTorah that we are informed that milk is no longer considered Eiver Min HaChai and is Mutar. Rav Sternbuch says we eat dairy meal on Shavuos to symbolize that.
But Rav Sternbuch’s reason is difficult. The fact is the we see from an earlier incident in the Torah that milk was permitted.
Avraham Avinu gave cheese and milk to his guests and did not worry about feeding them something which is forbidden to them. If dairy products were forbidden by virtue of Eiver Min HaChai he would have been guilty of Lifnei Iver Lo Setein Michshol, the forbidden act of putting stumbling blocks in front of others. Lifnei Iver applies to Jews putting stumbling blocks in front of non Jews equally as it does to non Jews. We see that milk was not considered Eiver Min HaChai or Avraham wouldn’t have served it.
But one can find in the Meshech Chachama a scenario that can be used as a defense of Rav Sternbuch’s reason. It is possible that milk served by Avraham was from a Ben Pakua. A Ben Pakua is a surviving fetus within an animal that was properly slaughtered. Such an animal is considered Kosher without Shechita. And as such, taking milk from the ‘Shechted fetus’ would not be considered Ever Min HaChai under these circumstances. And therefore, pre Matan Torah under normal circumstances milk was considered Eiver Min HaChai.
Rav Sternbuch is still difficult, however. If one looks at the Talmudic source of Rabbi Sternbuch’s reason, one will see an expounding of the words “land of milk and honey” (Shemos 3). If it were true that milk was considered Assur before Matan Torah because of Eiver Min HaChai, it would never have used it as an element in extolling the virtues of Eretz Yisroel. And this Pasuk in the Torah is written before Matan Torah during God’s encounter with Moshe Rabbenu at the burning bush. Since milk was used in this way, it shows that milk must have been permitted well before the revelation at Sinai, thus refuting Rav Sternbuch’s explanation.
Taken from Torah L’Daas.
The Art Scroll Gemoro in Bechoros 6b2 note 24 הואיל וליכא מידי דאתי מחי ושרייה דרחמנא
Since there is nothing that comes from a live animal that Hashem permitted.
note 24: There is a Bilical prohibition against eating the limb (or meat) of a live animale. We would therefore think that milk is included in this prohibition (Rabbeinu Gershom; see also Shach, Yoreh Deah 81:12 and Maharit Algazi 2:7) as the Gemara proceeds to state
והאי חלב כעבר מן החי הוא ושרי
yet this milk (of a kosher animal) is like a limb from a live animal and nevertheless it is permitted.
note 25: It is thus a novelty to say that the milk of a kosher animal is permitted for consumption (Rabbeinu Gershom)
The categorization of milk as a limb or meat of a live animal, however, is difficult to understand; while the limbs and meat are living parts of the animal, the milk is and independant entity held in animal's udder, and presumably not subject to the prohibitions applying to living parts of an animal (emphasis added). For this reason a number of Acharonim understand that the prohibition the gemara is referring to is actually not the classic prohibition against eating the the limb of a live animal (and thus not prohibited to a Ben Noach - my understanding of the quote), but is rather based on the fact that the milk is a product of a living animal lacking shechitah (see Noda BiYehudah, Tinyana, Yoreh Deah 36; Meromei Sadeh and other commentaries to our sugya). See Pleisi 81:7, and Chasam Sofer, Yoreh Deah 70 ד"ה נחזי; for further discussion of this issue see Kehillos Yaakov 6 at length.
וחלב דבהמה טהורה מנלן דשרי
Now milk of a kosher animal, from where do we know it is kosher.
note 27: I.e. why is it indeed not forbidden like a limb [or meat] of a live animal, or due to the fact that it originates from forbidden blood?
[*Shitah Mekubetzes 2 cites Tos. Chitzoniyos who ask why the Gemara does not derive the permissibility of milk from the fact that Avraham served milk to his guests (see Bereishis 18:8) See Maharit Algazi, Leisi (ibid.) and mitzpeh Eisan for further discussion.]
I think you are reffering to the gemara in בכורות ו׳ ע׳ב. There, the gemara is discussing a certain different limud according to one opinion and in order to understand his logic proposes a הוה אמינה that חלב כי אבר מן החי. The use of the word כי and the fact that this is never mentioned as actually being אבר מן החי anywhere would mean that it is in fact not. The following sugya discussing how we know milk is allowed are just proofs to the fact, not actual drashos.
Radak says that Avraham gave his guests the choice between milk and meat:
ויתן, he placed before them butter and milk as well as the young calf. He offered them the choice to eat either dairy or meaty.
So from this viewpoint Avraham wasn't doing anything wrong because his guests weren't eating milk and meat together, they were just given a choice which to eat.