Why do milk and orange juice taste so disgusting together?

Why things taste disgusting together is largely because they have nothing in chemically common.

You have your nose and tongue largely set up with receptors to respond to food.

However, when certain combinations of receptors are set of at the same time, such as with milk and orange juice, the receptors that are set off act in a similar way to a jarring musical chord: it tastes completely wrong.

On the other hand, if you play something smooth that goes together, they go together pretty well. So foods that have molecules in common tend to taste very good together!