Or HaChaim on Genesis 35:23/אור החיים על בראשית ל״ה:כ״ג

1 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.