Is this the first time he's done something like this?
I think if it is then i would try and work it out some men will realize when something like this happen oh smurf i could have lost the woman i really love and never do it again but if its not the first time then thats a big problem..
I'm not saying he is right, but if this is the first time find out why. A lot of times cheating *yes even with guys* is more then just a sex thing. If things are having issues at home, you are him are fighting, or he feels neglected etc, he might stray some. However if he has does this more than once, I say Leave.