Personally, I would feel that a lot of your guilt came from your wife, and you've apparently talked to her about your desire to be dominated. Perhaps you should try talking with her about your desire to be dominated a little more now. Open communication I think is one of the most important aspects of a relationship. There is a chance that your wife might simply feel relieved that you are getting your submissive urges supplied somewhere else, provided you still have sex with her.

And as for the orgasm, I've noticed with a lot of men as the guilt increases, the ability to control orgasm decreases. Psychologically, you felt as if you were cheating on your wife, the idea that only orgasm connotates cheating just means that you were feeling guilty already. The spanking punishment reinforced the idea that you were guilty and deserved punishment, and the punishment was making you feel better, hence orgasm. It was almost a foregone conclusion that mentally you would put yourself in the place to orgasm. Absolutely nothing to be ashamed of in that regard, in my opinion.

--Mali