One of the ponies that I went to see at the weekend was sold to his new owner with the small caution that he didn't like his undercarriage touched. I was asked to see whether I could help in this area! It's natural for horses to be protective of this region and ticklish when touched - they often dislodge flies by kicking at them and this is what he did to my hand too. Once again I found a flat, deep touch, in front of the relevant area made him less sensitive and that by keeping my hand there when he gave one reflexive kick and taking my hand away once he was still taught him that it wasn't going to go away. Later in the session I was able to have a much more exploratory search without him kicking at all and it soon became clear that he had a lot of greasy, waxy material in the pocket where he keeps his penis. Many horses suffer with this and geldings in particular. A good dollop of baby oil smoothed around this pocket at hand temperature can do a lot to make a horse feel far more comfortable and certainly happier about being touched. Some horses will need a more advanced clean particularly if they have a hard 'bean' of smegma wedged in the end of their penis. Talk to your vet if this is the case or read a good article on sheath cleaning.
"Many thanks for coming out to see us, it was a pleasure to meet you and I really found the lesson informative and useful. You have a very natural and amazing rapport with horses...You will laugh, but after [my ride] I tackled his man bits (I had bought some baby oil as you suggested) and he was as good as gold. Problem solved, thanks to you!" SH