rabbi shimon son of eliezer teaches that after leah died and jacob moved his bed to the tent of bilhah, reuben took the bed from bilhah’s tent and put in the tent of his mother leah to honour his mother’s memory and the sin reuben committed by doing that was compared to incest but reuben did not actually have sex with bilhah. though there are also rabbis in the talmud who say that he did.