Hi islesguy. BS here.
As a female, I agree with your BW - a woman wearing a thong is naked. I think you may believe that too, and possibly you used the 'they have thongs on' when you were going to this club to justify it in your head, so you're reverting to what allowed you to cross the line in the first place.
Why do I say a thong is the same as naked? It's my opinion, but a 2" x 3" piece of material held on by a piece of ribbon is the same as naked. Would you feel she was out letting naked men dance in her lap if she went to a club where they wore a small loincloth?
As a female, I can say that for a lot of us, a woman bouncing and grinding on your lap, wearing only a thong, is very insulting and degrading. Being in the clubs like this is bad enough. It makes us feel inadequate, no matter how we look. After all, why this need to go see other women? But a lap dance isn't just you watching from a distance. It's touching. It's way too close. It's just you and her. It hurts.
I think it's human nature, when confronted, to want to appear in the best light possible. It's hard to agree to things we have done or sad that we aren't particularly proud of when confronted with it. As a BS, that's what I needed from my WS. I needed him to admit to how bad his actions were, and why. I needed him to understand why his actions hurt me and how deeply they affected me. When he would defend his actions, it was hurtful because I felt that he still thought what he did was ok, and that he was minimizing my pain by acting as if his actions weren't bad, and he was saying (without saying) that I was over-reacting.
I've been told that anger is a secondary mask emotion. It's used to cover another emotion we don't want to show. In this case, I would bet that your BS feels insecure, not enough for you, not woman enough to keep you from wanting other women, and that in turn produces shame and embarrassment. No one likes to admit to 'not being enough' - even though I'm sure your wife is enough - and that's probably how she feels if you were visiting this club and getting lap dances. So anger comes in to protect her. She doesn't feel safe sharing her true feelings, so her subconscious is pushing anger to the surface to hide it.
That's my take on it. I could be very wrong. You know your wife better than I do. If there's a possibility that this is how she's feeling, I think trying to understand the pain she is in, and why, will go a long way. Once you're there, you can talk to her about how you feel about making her feel this way.
That's where I'd start. Whatever it is, I hope you're able to work through why you felt a need to minimize what happened, and then to help her understand it.