For you personally, you've found something to get out of your wife's hot wife lifestyle. To me that's critically important. No matter what anyone says, we live in a transactional society. Everybody gets something for having given something. When it gets difficult is when it becomes one-sided enough to foster feelings of disparity which leads to resentment. If you can perceive an "ROI" for your part in your wife's activities then you have a motive to continue. If you don't see any tangible ROI then what's the point? Sure, she may be getting much more out of it than you are BUT as long as you are getting SOMETHING out of it then it's worth it until it isn't.
What is crazy is believing ANYONE that says they don't want any sexual satisfaction of any kind at all AND is in a committed relationship. Why would you be with anyone that doesn't acknowledge that EVERYBODY has some form of sexual desire and want no matter how perverse or extreme or how vanilla? If you know your partner (the one you love with all your heart and soul) wants some kind of sexual experience and you intentionally deny it then what does that say about you?
Regarding that husband of your wife's friend, my guess is that he's getting a thrill out of his submissiveness. This will only last as long as he gets some form of sexual attention. Once it becomes so routine that his compliance is certain, the thrill will be significantly less for everyone. It simply won't be fun anymore because he's been broken, there's no resistance in his heart. His hot wife won't see him as a challenge to manipulate and use, she'll get bored, she'll see him as predictable and uninteresting. Even if you're a limp dicked, unattractive man you still have sexual desire and thoughts.
I predict that if your wife's hot wife friend doesn't catch on her husband will reach a tipping point and he'll see there's nothing (literally nothing) there for him, so why stay? Bottom line, it may not be equal, it may not be as good, but everybody needs to get something out of a relationship. If you have convinced yourself that that is unnecessary you are out of touch with reality.