They lie because they really do not want to damage the potential of a relationship with you if or when they ever decide to consider the LBS as a viable option and want to return...and they've done so many things that they are deeply ashamed of".
Adia,
I agree. My H has demonstrated both of those ideas to a tee.
Many LBS's express confusion as to how their spouses can know they are making regrettable, hurtful decisions, and still plow on. One word: ADDICTION.
I've been addicted to substances, and had an eating disorder, and I know what it feels like to be caught in compulsive behavior I know is destroying me. I also know the freedom of facing one's demons and breaking the addictive cycle.
I was in rehab in the early 90's. I was one of the few patients who'd never had a DUI, or any trouble with the law, and who still had a viable career, good health, and relatively undamaged relationships with family/friends. I remember looking at the many cohorts who'd lost everything, and thinking, "How will they ever get sober? They've destroyed their lives. They have nothing to go back to."
Just writing this gives me great compassion for H, and the desire not to abandon him, at least as a loving friend. I think it also speaks to his lying...he doesn't want to 'blow everything up' because he hopes one day to recover and 'have something to go back to'. Of course, as with any addict, I need to detach and set boundaries. Like OP says, Al-anon is a great help in doing this.
I see H's OW as a big pile of cocaine. Thrilling for the moment, but a bad, bad ride over time.