What you're describing is not uncommon. It sounds like you don't actually want to act on the fantasy of your wife with another man, you are just turned on by the thought. Even the fantasy about the paddling is not surprising and I don't see anything wrong with it.

I go into these fantasies in my mind while I am having sex with my husband. At first I felt bad about it, like I was cheating by not totally thinking about him. I told him about them. He is fine with it and we think it's probably very common and that most women do this. It's totally harmless and it gets me off.

I'm not sure of your age and that may be a factor in your embarrassment and reluctance to confide in your wife. You say she is a 'good woman'. Does that mean you think she will be shocked by your 'kinkiness'? She's your intimate lover. You should be able to share your thoughts with her.

I would not try to repress myself from the fantasies and see nothing wrong with having them. I think the one about the paddling would be the best one to tell her about first, if you want to. Even if it comes down to the thought that you were subconsciously aroused when you got paddled as a boy, I see nothing wrong with that and don't think it is at all unusual.