Kommentar zu Bereschit 35:23

בכור יעקב JACOB’S FIRSTBORN — even when it speaks of his corrupt action Scripture calls him firstborn (Genesis Rabbah 82:11).

בכור יעקב ראובן, as far as Heaven was concerned Reuven did not suffer consequences because he had repented forthwith. He also did not suffer really in this world, i.e. on earth, until on his deathbed Yaakov punished him by depriving him of the material benefits enjoyed by the first born in that he inherits an extra share of his father’s estate. This punishment was withheld in accordance with the principle that anyone guilty of being ostracised by the community is not actually ostracised until the leading sage of the generation declares him to be so. (Moed Katan 16) This is based on Judges 5,23 where Devorah, the ruling judge of Israel, curses the inhabitants of a certain place called מרוז who had failed to answer her call to arms against the army of Siserah, general of Yavin, King of the Canaanites who had been oppressing the Israelites. The מלאך ה' she quotes in that verse is the leader of the generation, herself in this instance.

בני לאה, בכור יעקב ראובן the sons of Leah Jacob's firstborn Reuben. The reason that the Torah had to repeat here that Reuben was Jacob's firstborn was in order that we should not think that Reuben's misconduct with Bilhah had disqualified him from being the firstborn. This also proves that when the Torah described Reuben as having slept with Bilhah this was not meant literally. When the number of Jacob's sons is described as 12, this too is proof that Reuben had not committed an act by which he forfeited being included in that count. Inasmuch as Reuben represented the first drop of semen Jacob ever ejaculated, the idea that he could have been guilty of such a horrendous crime as actually sleeping with one of his father's wives is quite impossible. This is why the Torah emphasises בכור יעקב, he was Jacob's firstborn.