The horror here guards the boundary that makes a family, and therefore a culture, possible at all. To take the father's wife is to collapse the distance that holds generations apart, and that distance is the scaffolding on which every ordered thing is built. Where the son overthrows the father by seizing what belongs to him, the structure that carries life across time dissolves into chaos.
The penalty measures the stakes. Incest is the archetype of the catastrophe that follows when the categories separating parent from child are erased, when appetite tramples the barriers civilization depends on. To uncover the father's nakedness is to unmake the order that raised you. A society that will not defend its most basic boundaries has already begun to come apart.