Ambrose of Milan

Doesn't this seem to be a reproach rather than a blessing? Thus it really is more a prophecy than a blessing. For a prophecy is an announcement of events to come, whereas a blessing is the longed for bestowal of sanctification and of graces.The Jews suppose that the old man is saying these things to his son Reuben on this account, because the latter lay with Bilhah, his father's concubine, and polluted his father's bed. But they are easily refuted; this had already taken place. …

On the Blessings of the Patriarchs ↗

Bonaventure

Carnal concupiscence took from us our noble generation, by which we would all be born innocent and heirs of paradise, and it makes us be born in death and guilt and the loss of the birthright. This was prefigured in Reuben, who lost the right of the firstborn, because he went up to his father's bed and defiled his couch; and Joseph, on account of his chastity, was made prince over his brothers.

Collationes de Decem Praeceptis, Collation 7 ↗

Hippolytus of Rome

First he mentions the event,-that in the last days the people will assault the bed of the Father, that is, the bride,9 the Church, with intent to corrupt her; which thing, indeed, it does even at this present day, assaulting her by blasphemies.

Hippolytus Exegetical Fragments ↗