Geldings are not natural to the evolution of equines! We have made them geldings so it is easier to ride them. There are no geldings in the wild by nature! They do not clean themselves by mating. They are not like cats and dogs that clean themselves.
We have domesticated these animals and the environment and work we keep them in all encourage a build up of smegma! They develop beans which is hard smegma in the end of the Penis! So you may look at your horse and think his manhood is not that dirty but there is far more than you can see when he dangles it out.
I recommend that your gelding is cleaned yearly unless advised otherwise. With water soluble lube and no chemicals that will irritate his sensitive skin.
Dirty Willy
Smegma on inside of the legs
Itching tail or tummy
Swollen sheath
Change in behaviour
Trouble urinating