So you and your wife have two daughters, right? 2 1/2 years and 4 months? Kids really change the husband/wife dynamics. Not to mention, marriage really changes couple dynamics.
Affairs are free from responsibilities. That's part of the allure: all the baggage that goes with being married gets thrown out the window. The affair is about passion without responsibility. Passion without all the other stuff that goes with living with someone and building a life together. Responsibility-free passion can be very attractive.
Some people sort of idealize marriage. Especially after kids enter the picture. They want family life to be wholesome and safe. And in their minds, SEX doesn't fit into this picture very comfortably.
This is particularly problematic for people like my own wife who consider sex to be dirty. She's not a prude. But she identifies sex with decadence ... and that has little place in family life. We'll go out on date night. We'll hold hands. We'll kiss. We'll do everything that indicates we're very close. But when we get home ... my wife falls asleep. She has almost no interest in sex. Because in her own words, it makes her feel creepy--especially when our daughter is sleeping only one room away.
Marriage and children can really dampen the passion in a marriage ... That's not terribly profound. I'm not telling you anything you didn't already know. But people respond to family life in different ways.
It sounds like you're taking good actions. Reading SSM together is an excellent step. It's very encouraging that your wife is willing to do that. Very encouraging. Another good book is PASSIONATE MARRIAGE by David Schnarch.
me: 50 w (waw): 45 daughter: 9 m: 16 t: 19 bomb: 9/26/08 status: physical separation for 7 weeks, then work-in-progress R