remember something...
everything may seem rosy with the girlfriend- but she's not the one doing your laundry or making you dinner or helping to pay the bills, etc. Her sole job as a girlfriend is to listen to you complain about your terrible marriage, and to be the sexy, pretty thing you wish you were married to.
both of those things are easy for her, because she has an agenda. Your wifes agenda, on the other hand, is keeping your family safe and taken care of, and nomatter how much you may think having a gf isn't affecting your marriage... it is. and now she has to deal with that, even on a subtle level. Even if it means you just have a lack of sex drive or interest in what she has to say or how she feels- she's going to know it and react to it.
all your gf has to do is listen to you when you complain about your wife reacting.
it's easy to paint this picture of your future with your girlfriend, always being happy to see you home, the two of you screwing like rabbits then frolicking outside somewhere... but when its her turn to clean up after you or she's running up your credit card or going through your emails trying to find out what your up to, you're going to realize the evil that you know is often better than the evil that you don't.