Two questions:
1) The gemorah's second side is to say that one cannot prove from Har Sinai that Tumas Keri is "roeh", since we find that the even more chamur tumos of Zav and Metzorah were not precluded, as was Tumas Keri.
Question: all illness was cured before Kabals Torah at Har Sinai, which presumably includes Zav and Metzorah. If that's the case, one can indeed prove that Timas Keri is "Roeh", as the disproof from Zav and Metzorah is no longer a disproof- those Tumas were not preculed b/c they didn't exist- as they were cured, but had they existed, they too would have been precluded.
2) The two sides of the coin that the gemorah considers do not seem to be two sides at all. Had the two sides been either: (a) Tumas keri is roeh b/c we see that (as Rashi points out) that tumas negiah of sheretz was not precluded at Sinai OR no, we see that Tumas roeh of Zav (specifically, but do not cite Metzorah, nor do not cite the relevance of being more chamur) was not precluded from Sinai, so therefore, one cannot conclude that Tumas Keri is Roeh by virtue of Tumas Keri being precluded at Sinai; (b) Tumas Keri is roeh b/c we find that this tumah was chumar enough that it was precluded at Sinai- therefore it must be roeh OR no, we see that even more chamur Tumos were not precluded at Sinai (specifically citing BOTH Metzorah and Zav and the relevance that these Tumos are more chamur than Keri) , so therefore, one cannot conclude that Tumas Keri is Roeh.
The gemorah, as it stands (and the way Rashi explains it) seems to be a hybrid of both of the above approaches. This is puzzling to understand since when evaluating which of two sides of an issue should be taken, it normally doesn't make sense, when addressing the second side, to switch gears and head off into a completely different lien of logic (instead of debating the two sides of the logic presented at hand).
Can you please help shed light on this?
Daniel Gray, Toronto, Canada
1) The Rashash asks your question, and does not supply an answer. The Aruch Laner answers that even though they were cured by Har Sinai, they would still need to count seven days before they became Tahor. It is therefore still a Chidush that the Torah was Makpid on Shichvas Zera but not Zavin u'Metzorain.
2) The lien of logic seems to be focused on "stringencies of Har Sinai" (similar to the second approach you presented above). Do we learn from the stringency of Ba'al Keri of Har Sinai and conclude Ro'eh, or do we say that the stringency of Har Sinai cannot be learned from, as it was clearly not a regular Halachic stringency due to the exclusion of Zavin and Metzorain? The logic seems obvious: while understandably a Chidush, can we learn from it or not? I think that the first approach excludes the important argument that Har Sinai possibly cannot be learned from at all, as it has the anomaly of excluding Zav and Metzora while being stringent on Ba'al Keri. It is very possible that the Gemara wanted to bring this out, and therefore did not present the question as you suggested.
All the best,
Yaakov Montrose