When men feel pain that other men will, or have felt, whether this be in real life, on the internet or watching various games of sport on TV.
But menpathy is not restricted to pain to the genital area. It can be other things that men experience (such as being cock-blocked or missing out on something that men would enjoy.)
Menpathy is simply empathy between men.
*Man gets kicked in the balls*
Man 1: OWWWW *holds balls*
Man 2: That is a serious case of menpathy.
Man 1: I have the disc to install Skyrim but I can't do that until it is realised tomorrow.
Man 2: I feel your pain, man.