How does a guy feel when he is cheated on?

Unless his heart is as hard as a rock (which could be the very reason one would cheat on him), it's going to be a mixture of these things, which do not differ much from what anyone would feel:

*Surprise/Shock - Getting hit by a freight train - Wind knocked out of you

*Betrayed to the utmost (More wind knocked our of you)

*Heart-Broken

*Sadness/loneliness/anger

*Any/all stage(s) of the grief process

Depending on the guy, your end result:

Heart as hard as a rock, Bitterness, Apathy, or, possibly, forgiveness, openess to a second chance (tread lightly as trust has to be rebuilt from the start again)