Why is the term "incestuous fornication" applied to an act that may be committed under duress, knowing that fornication is presumed to involve the consent of both parties?

Incest is one of the most heinous sins. If it occurs by mutual consent, then both parties are to be executed, whether they are virgins or previously married, according to many scholars. However, if it occurs under duress, then the punishment is applied to the one who coerced, and there is no punishment or sin upon the one who was coerced. Thus, consensual fornication is not like fornication under duress, even though both are called fornication, their rulings differ.