Defcon,
Think about what you are saying here, and the circumstances you were in during that moment (the drive to her place).
* You were going to see a woman who wasn't your wife
* You hid the fact that you were going to see this woman from your wife and everyone else
* You took steps in order to make the time and circumstances required for this to happen
* You made lies/excuses about where you were and what you were doing and why
* You decided that it was worth the time, cost and effort it took to lie and hide this relationship, and it was worth it to keep it up for a year
* You must have had some communications with this woman before this meeting, and the two of you had gotten to know each other well enough that you both agreed to plan a secret meeting together. Alone. In her house.
* Did you bring protection? Be honest.
* Was flirting going on? Be honest.
Sorry bud, but I don't see how anyone could believe that you went over there expecting to play Tiddlywinks with this woman. Maybe you didn't expect sex first thing, but be honest with yourself. What you did want to happen? Were you hoping for a kiss? Did you want to get closer to her? Let's be clear in that you weren't going over there in order to improve your marriage.
One thing I found (I am a WS as well) is that I often focused on verbiage and minutia in order to avoid the truth. For example:
Wife: You went there to have sex with her all day long!
Husband: That's not true! We only had sex once.
Again, you hid, lied, planned, and made efforts to go see a woman, in private, in her home, alone, without telling your wife, and you are expecting her, and us, to believe that you didn't even consider what the possible desired outcomes would be? Sorry. No. That's not the truth. That's protecting yourself from the truth.
I'm not denying that there may have been some compartmentalization going on. As you said, guilt is often covered up by avoidance. "If I don't actually plan it, then it isn't real". Sort of like how a person on a diet might eat a bowl of ice cream, but it doesn't "count" because no one saw them eat it. It's just a mental game. The point is, you know it counts.