It’s an illegal desire, a desire which isn’t allowed in the Shari’ah. It can be towards anything; same gender, opposite gender, human, non human and so on.
This is a disease of the heart. As the diseases of the heart are very very dangerous and very widespread.
This shows that just because a person has a beard and prays salaah, doesn’t mean they are not free from these diseases. A person who is just praying his salaah is a Muslim. That’s it. It doesn’t mean he’s practicing.