a disorder of structure or function in a human, animal, or plant, especially one that produces specific symptoms or that affects a specific location and is not simply a direct result of physical injury.
Per this definition, homosexuality is a disease, it is one of many disorders that changes how the human body functions in certain situations.
Now, there are three causes, Amore, Luxuria and Blanditia.
The first symptom can manifest as a turmoil in your tummy,heart Palpitations, and loss of appetite.
Luxuria symptom that causes an intense need to do coitus.
Blanditia has so many symptoms thats its pointless.
Now, Amore is Love, not everyone can love, but most can, it is curable, but the cure causes immense pain.
Luxuria is Lust, not everyone is capable of lust, it is curable, but the cure can cause intense shame, or a healthy glow and a happy step.
The last one, Blanditia is Attraction, this is something everyone can feel, there is no known cure.
Now, if you see Love or Lust or Attraction as unnatural then you should see a doctor, cause you might have an issue.
As you see, this disease just happens, there is little you can do about it.
Other diseases with the same symptoms include Bisexuality, hetrosexuality and many many more.