"One must not force nature but persuade her. And we will persuade her by fulfilling the necessary desires, and the natural ones too if they do not harm [us], but sharply rejecting the harmful ones".
- Epicurus, Principal Doctrines 21
Obviously, the harmful desires are the non-natural ones based on other peoples opinion. These are also called "groundless" by Epicurus. What makes them harmful is that they are obsessions and as such they will in themselves do damage to our peace of mind but they are also likely to lead us to damage our personal relations, our health, our job and our finances. Just look at Harvey Weinstein. From an Epicurean point of view, the fall of Harvey Weinstein was not caused by him having strong, unfortunate natural desires but by the fact that he grew up in a sick culture which teaches men that it is a good thing to "bang a lot of chicks". The desires which drove him to ruin is - from an Epicurean point of view - quintessential unnatural, harmful desires.