I Say Men Will Be Men

Last night I found out the guy I've been seeing for the past month is married. His defense is that his marriage has gone bad and he'll probably ask for a separation soon and I never asked him if he was married so technically he's been honest with me. That's total shit. He deceived me like a coward.
It hasn't been that long so I don't have a huge emotional investment but I was really feeling like it might go somewhere so today I feel like someone ran over me. How do I trust guys when they do something like this?