Catechism number 1755

A morally good act requires the goodness of the object, of the end, and of the circumstances together. An evil end corrupts the action, even if the object is good in itself (such as praying and fasting "in order to be seen by men").
The object of the choice can by itself vitiate an act in its entirety. There are some concrete acts - such as fornication - that it is always wrong to choose, because choosing them entails a disorder of the will, that is, a moral evil.