Okay, so this is how it seems like the correct translation of this pasuk is:
that Reuven went and lay with Bilhah, his father’s concubine…” (Bereshit 35:21-22) That would be an unbelievable act if it were literally true. Therefore, Rashi, based on the Gemara in Shabbat 55b, explains what happened as follows: “…When Rachel died, Yaakov moved his bed, that had been placed in the tent of Rachel, into the tent of her handmaiden, Bilhah. Reuven came and contested this shaming of his mother. He said, ‘If my mother’s sister was her rival, that’s one thing, but that her handmaiden should be a rival as well, that is intolerable!’ Therefore, he moved his father’s bed to his mother’s tent.” (Rashi 35:22, text beginning “He mixed up”) Because Reuven entered into matters that were none of his business, the Torah used such strong language to describe his act.
Does anyone (any perush...) interpret it literally, that Reuben slept with Bilhah?