I think they have little control over "it". I don't think it's anything to worry about. I am sure other male GSD owners can offer better advice.
Funny story: I have only ever had females...so I was rubbing the belly of my parent's male border collie. He liked it... I was grossed out and laughing when I realized what was happening. I had never hesitated to give the girls a belly rub. I have to be more careful when I rub Dillon's belly now.