Why don't I feel wrong about him cheating?

You're not getting off on this guy. You're getting off on the whole idea of this being a secret affair - it's an adrenaline rush. You're doing something illicit, dangerous and adventurous. You know you're going to get caught but how long can you get away with it? It is thrilling and definitely heightens the ual excitement of the situation but you're not thinking beyond that moment. You know the g/f of this guy is going to be hurt. Put yourself in her shoes. How would it feel to find out the love of your life was cheating on you? Also, what are you doing to your own reputation? This is taking place at work so when everyone finds out, you're going to lose respect from your bosses and co-workers which probably won't help you get a promotion or pay increase in the future. Finally, think about your own self respect. How do you feel when you look at yourself in the mirror (and I'm not talking about how you feel getting your lust satisfied)? That little inner voice of your conscience is already telling you that this is wrong and you should stop. Just because something feels good doesn't make it good for you. People become drug addicts/alcoholics because they like the feeling they are getting and think that they can't/don't want to stop until they end up ruining their lives over getting that feeling. You can stop this before it gets any worse and you'll have more respect for yourself if you do - not to mention that the g/f may not have to find out and be hurt. You just have to make the decision that your own pleasure is not the most important thing in this situation.