When an act is intrinsically evil, it is immoral by the very nature of the chosen act. The moral nature of an act is its inherent moral meaning before God.
An act is a choice; it is an exercise of free will and intellect. An act is a deliberate knowing choice. All acts are subject to the eternal moral law, even if the act is entirely internal (within the confines of the mind and heart).
The moral nature of an act is determined by its object: the end, in terms of morality, toward with the knowingly chosen act is inherently ordered. This inherent ordering of the act toward a good or evil end is what makes the act intrinsically good or intrinsically evil, that is to say, good or evil by the type of act.
Intrinsically evil acts are immoral regardless of the intended end of the person choosing the act, and regardless of the circumstances, because the type of act being chosen is wrong in and of itself.
Most Catholics do not accept this teaching.