Zera Shimshom ..... Ki Tavoi ........

And these shall stand for the curse on Mount Eival: Reuven, Gad and Asher...

Cursed is he who lies with his father’s wife... 

The Gemara in Shabbos establishes Reuven’s innocence from the fact that his descendants were among those who were chosen to stand on Mount Eival and affirm the curses.

That righteous person [i.e. Reuven] was clearly saved from that sin [that the Torah seems to attribute to him; lying with his father’s wife, Bilhah], as it’s certain that this deed could not have come to his hand, for is it possible that his descendants would in the future stand on Mount Eival and declare, ‘Cursed is he who lies with his father’s wife’, if this very sin had come to his hand?

[i.e. For if Reuven had actually laid with Bilhah, Hashem would surely not have selected his descendants to be among those affirming this curse]. 

The commentaries ask on this Gemara; why the need to establish Reuven’s virtuousness from this incident which happened a short time before Moshe Rabainu’s death, when it could have been proven from a much earlier indicator.

For the Gemara in Sotah says the following;

At that moment [before Yosef succumbed to the sin], the visage of his father came and appeared to him in the window and said to him, “Yosef! Your brothers are destined to be inscribed upon the stones of the Efod, and you are destined to be among them. Do you want your name to be erased?”

In view of that, when we find Reuven’s name inscribed on the stones of the Efod, that in itself should clearly attest to his integrity, for had Reuven indeed sinned, 

his name would have been erased from the Efod; just as Yaakov had warned Yosef would happen if he would sin.

Truthfully, we can pose a much greater question on the proof that the Gemara chose to use when establishing Reuven’s innocence.

For immediately after the Torah narrates the incident which seems to indicate a wrongdoing on Reuven’s behalf, the Passuk proceeds and says,

- and the sons of Yaakov were twelve, and Rashi cites the Chazal who say,

– These words come to teach us that all Yaakov’s twelve children are equal and all are righteous, for Reuven did not sin.

Accordingly, we need to understand why the Gemara would need to prove Reuven’s innocence from the incident which occurred on Mount Eival, when the Torah itself plainly and clearly vindicates him of any sin? 

This obvious difficulty would compel us to say that that those words alone, where the Torah acknowledges that all twelve sons were equally righteous, would not suffice to prove that Reuven didn’t sin, as we can easily say that Reuven did indeed sin, yet only because he repented was his sin atoned for, and thus was he once again considered righteous, equal to his brothers.

Accordingly, we can similarly say that from this fact alone, that Reuven’s name was inscribed on the stones of the Efod, we could not have proven that he had never sinned, for by the same token we can say that he had indeed sinned, and only because he repented was his name eventually inscribed on the Efod stones.

We can now understand why the Gemara needed to prove Reuven’s virtuousness from the fact that his descendants were chosen to stand on Mount Eival and affirm the curses, and not from the fact that Reuven’s inscription was on the Efod, nor from the fact that Reuven was considered equally as righteous as his brothers.

For if he had indeed sinned, then although his repentance would have sufficed for him to be considered righteous and to be reinstituted in having his name inscribed on the Efod, nevertheless, out of respect for his honor, Hashem would still not have chosen Reuven’s descendants to be among those selected to affirm the curses which the Torah places upon those who violate the Torah’s commandments.