The "Sin" Of Reuvain

The Gemarah Shabbas (55b) says כל האומר ראובן חוטא אינו אלא טועה.  So why does the Torah write that Reuvain did a grave sin?  Rav Ruderman (Yeshuran volume 17) explains that every issur contains many layers to the prohibition.  There is the actual translation of the words prohibition and then there are degrees and levels to the prohibition.  As an example he says there is an issur not to kill and Chazal equate embarrassing another person to killing them.  In other words, to a degree, embarrassing a person is included in the issur of killing.  With this idea he answers a contradiction.  Rashi in the parsha (37:7) says וכן לא יעשה שהאומות גדרו עצמן מן העריות ע"י המבול.  He says even gentiles were careful about arayot.  But the Midrash  (V'zos Haberacha) says that Amon and Moav didn't take the Torah because they couldn't accept לא תנאף, but they kept the issur of arayot?  They kept the basic law of not having a forbidden relationship but all the extensions of the issur, הרהור, הסתכלות, וכו that they did not keep.  This is the peshat in the story of Reuvain as well.  According to his level his act was an act that was a degree of the issur of לשכב פילגש אביו.  He says this is the yesdo to explain many of the sins mentioned in Tanach.  

Rav Neventzal is מדייק the Gemarah doesn't say האומר ראובן חוטא טועה, that person is wrong for thinking Reuvain sinned but the Gemrah is very verbose and says  האומר ראובן חוטא אינו אלא טועה, why the extra words אינו אלא?  He explains the Gemarah is telling us that one is not just making a mistake in understanding a possuk but one's entire philosophy, one's entire outlook on understanding Tanach is wrong.  אינו אלא טועה means the entire person, his whole approach is mistaken.  Why?  Because it is self understood that such great people can not stumble in such a depraved an immoral sin.  Reuvain, who's name in engraved on the choshen, could not possibly have violated such a great sin.  Someone who doesn't understand that does not know how to read Tanach properly.  So, why does the Torah write it in such a manner?  He gives a משל from a precious diamond that even a small scratch makes the value diminish greatly since the more precious the item is, the greater the perfection demanded from it.  One will tolerate a small scratch in a regular stone but in a precious diamond than more is at stake.  So too for the great figure of Tanach, even there small miscues are cast as great sins for according to their level any lack of perfection taints to the level of a great sin for an ordinary person.