Explanation #1 (Part 1): So that they should not say 'We have come from a stricter Kedushah to a more lenient one!' ...

Explanation #1 (Part 2): Because R. Meir holds that a maternal sister is forbidden to the B'nei No'ach, and even a maternal sister of his father or of his mother, as R. Eliezer will learn later from the Pasuk "Al-kein Ya'azov Ish es Aviv ve'es Imo".

Proof (Part 1): This explanation is borne out in the second Perek of Yevamos, 2a & 22b, where they asked in Eretz Yisrael whether the Chachamim decreed Sheniyos by Geirim or not ...

Proof (Part 2): And the Gemara replies 'Now that Ervah, if not for the concern that they may say 'We have come from a stricter Kedushah to a more lenient one!', they would not have issued a decree forbidding it, how much more so Sheniyos!'

Query (Part 1): Rashi in Yevamos, 97b however, ascribes the ruling that a Ger is obligated to divorce his maternal sister to the fear that he might then marry a sister who is born later, which is an Isur d'Oraysa ...

Query (Part 2): ... implying that there where both the pregnancy and the birth were she'Lo bi'Kedushah (when he was a Nochri and), where there is no question of an Isur d'Oraysa, his maternal sister is permitted.

Refutation: It is impossible to say this however, seeing as the Gemara says there with regard to the sons of Yudan the maidservant who were set free (where both their pregnancy and birth were she'Lo bi'Kedusha), yet Rav Sheishes forbade them to marry each other's wives.

Question (Part 1): However the Gemara concerning the sons of Yudan the maidservant that we just cited, implies that the reason for the prohibition is not because 'Shema Yomru' (they may say 'We have come ... '), but because of 'Ichlufi' (one might come to confuse them with a Yisrael) ...

Question (Part 2): Since the Gemara says there that everyone agrees with regard to maternal brothers are prohibited, and that the dispute there is confined to brothers that share both the same father and the same mother ...

Question (Part 3): Now if the reason was due to 'Shema Yomru ... ', on what grounds would anyone permit the latter case?

Question (Part 4): This is not a problem if the reason is because of 'Ichlufi' however - because, seeing as in the current case there is a relationship from the father's side, and a paternal relationship is not applicable to Nochrim, the decree will not apply.

Precedent (Part 1): And from the Gemara in the second Perek of Yevamos, 22a too, it would seem that the reason is because of 'Ichlufi', because Ameimar there permits even maternal brothers to testify, and the Gemara goes on to attribute the difference between testifying and Ervah (where they are forbidden) to the fact that 'Ervah is handled by everyone, whereas terstimony is handled by Beis-Din' ...

Precedent (Part 2): Implying that the reason by Ervah is because of 'Ichlufi' (which applies to Beis-Din) and not because of 'Shema Yomru' (which does not).

Support: And indeed, Rashi there comments 'Ervah is handled by everyone' - Everybody marries women, and if one permits them to remain married, they will also come to permit Arayos to a Yisrael'.

Answer (Part 1): In fact, a Ben No'ach is not forbidden to marry even his maternal sister (min ha'Torah), and the two reasons are both needed, since there are times where one of the reasons is applicable and not the other, such as someone who marries his maternal sister, where 'Shema Yomru' is not applicable, whereas 'Ichlufi' is.

Answer (Part 2): On the other hand, his father's wife according to R. Akiva, and his father's sister according to R. Eliezer, which are prohibited to the B'nei No'ach, which we will forbid due to 'Shema Yomru', but not on account of 'Ichlufi', which is not applicable in cases of paternal relationships (since everybody knows that the Torah rendered Hefker the seed of a Nochri).

Answer (Part 3): Whereas his father's wife according to R Eliezer, and his father's sister according to R. Akiva, are permitted - seeing as neither 'Shema Yomru' nor 'Ichlufi' applies.

Explanation #1: When the Gemara asks from R. Meir on R. Meir, Rashi explains that it is also asking from the fact that he rules 'Asur ba'Achoso va'Achos Aviv va'Achos Imo', even though Beis-Din shel Yisrael do not sentence to death an account of them.

Refutation: This is not correct, seeing as we could have answered that all three cases are not forbidden because they were forbidden whilst they were still Nochrim, but because of 'Ichlufi', like the Gemara says in the case of Achoso.

Explanation #2: In fact, the Gemara is asking on R. Meir from his ruling 'Mutar be'Eishes Aviv u'va'Chamoso', even though Beis-Din of Yisrael sentence to death on account of them.

Support: And this is evident in the standard text above, which reads 'And does R. Meir really hold that every Ervah which Beis-Din sentence to death, the B'nei No'ach are warned? Have we not learned in a Beraisa ... '?

Conclusion: A clear indication that it is from 'Mutar be'Eishes Aviv u'va'Chamoso' (exlusively) that the Gemara is asking.

Clarification (Part 1): When Chavah ate from the Tree of Knowledge, she deserved to die immediately ...

Clarification (Part 2): As for Adam remaining without a wife, he could have married his daughter.

Conclusion: The Gemara concludes that the reason that this didn't happen, was to enable Kayin to marry his sister.

Inference: This implies that a man is permitted to have unnatural relations with his wife.

Precedence: And this is borne out by the Gemara in the second Perek of Nedarim, Daf 20a, in the case of the woman who came before Rebbi and told him that she had 'prepared the table' for her husband, but that he had overturned it. To which Rebbi replied that the Torah permits him to do that (and that there was nothing he could do about it).

Clarification (Part 1): She cannot have been referring to them changing positions (she on top and he underneath), because on what grounds would she then have objected? Consequently, she can only have been referring to unnatural relations, which causes the woman pain, as the Gemara says there, citing Lavan, who said to Ya'akov "If you afflict my daughters" (with reference to unnatural relations) ...

Clarification (Part 2): ... and as the Gemara explains with regard to Sh'chem and Dinah "And he lay with her", 'naturally', "and he afflicted her", 'unnaturally'.

Question: At the end of the third Perek of Yevamos, Daf 34b, the Gemara equates unnatural relations with the sin of Er ve'Onan?

Answer: That is only if one indulges in that sort of behavior constantly, to prevent one's wife from becoming pregnant, so as not to spoil her beauty, but if he generally performs a natural Bi'ah, and unnatural Bi'os only on odd occasions when he has an urge to do so, it doesn't matter.

Explanation #1: Rashi explains that he was accustomed to striking his friend, and that cutting off his hand is a penalty for doing so.

Explanation #2: One can also learn that it is a legal punishment, seeing as we learn it from the Pasuk "u'Zero'a Rasha Tishaver".

Proof: In fact, the Gemara in 'Kol ha'Yad' (Nidah, 13b & 14a) explains that the MIshnah "The hand off the person that touches his penis should be cut off' is Halachah, based on Rav Huna, who actually carried it out. Presumably here too, the ruling is Halachah.

Question: Then why did Rav Huna himself not cut off the hand of Chanan Bisha for striking his friend, in keeping with his ruling here?

Answer (Part 1): Here is different, because it speaks where the culprit is accustomed to striking his fellow-Jew (like Rashi explained), and not just once.

Answer (Part 2): Similarly, the Gemara in Nidah is speaking about someone who touches his penis constantly.