My wife and I have played in the lifestyle. Currently, we are on hiatus (meaning we are concentrating on other parts of our lives).
Before reading this, please take into account that I am generalizing--heavily. Everyone has different life experiences, etc. However, the parts about me are not generalizations
My wife and I would fall more into the hotwife and not the cuckold category, although I think it is a continuum and not an either or issue. As for me, I am not into humiliation by anyone. The blatant machismo aspect seems to be a bit retarded and a sure fire way to get me to snap out of the fantasy. However, I absolutely love to hear my wife talk about the bigger men she has been with (that would include most others), what the physical act felt like, the raw sexual passion, etc.
I am very secure in my sexual abilities, despite being below average in size. I have repeatedly brought many women to their first orgasms, first multiples, squirting, continuous (2 minute plus) orgasms, etc. (Most women love oral, fingers and most importantly: a gentle, sexy voice, and a great mind). Most women prefer to be enveloped, desired and consumed and chose the long term prospect of that feeling over size and the raw sexual power size alone provides.
When I was younger and before I could adequately gauge my smaller size in comparison to other men (or even knowing if and when I would reach my maximum size), I read and saw a lot of pornography. I could tell I was substantially smaller than the men in magazines. However, that did not abate my desire to fulfill a woman's desire as I had seen in magazines. At 18 or 19 I had figured out I would never hear a woman say "Wow" or "You're huge!" when taking off my pants, so I realigned my goals to having women say "You are incredible!", etc.
As an aside: You can tell when a woman is lying...some women know how ego shattering it is for a man to realize he is not endowed and go the opposite route to try and pretend you are big. Always awkward. It is as if they mention that an imaginary chocolate cake is in the room. It makes you wonder if they are more in the mood for chocolate cake.) When you are younger, you may be avoiding an ego-shattering event with a younger man, but after a certain age, it makes a woman seem very manipulative.
Most (not all women) respond more to confidence and technique than to just size alone. I am not saying larger endowed men and technique are mutually exclusive. However, I really started working on my other skills earlier in life with more zeal than most naturally gifted men. I had nothing else to fall back on. On the other end of the scale, I think men with sideshow-sized cocks face this same problem and develop their other techniques earlier as well.
So now that you know a little bit about myself, I can explain why the hotwife/bigger guy experience is such a turn-on. When reading those magazines all those years ago, I would see women overcome with desire when being filled by large men. As a young man, when fantasizing (either at the time or later on), I would imagine that I was the one with the large penis, taking her to places she had never been before, being the biggest one she had ever seen, etc.It is natural when reading pornography to take the stories and inject yourself into them.
Well, those fantasies became deeply rooted during my formative years. Because I can not actually physically perform them myself, I compromise with my fantasies with a surrogate. It is not his cock that is a turn-on to me, it is the fantasy of what it does to my wife and women. I don't fantisize about pleasing men. However, the idea of putting a large cock into my wife harkens back to these early, almost ingrained, fantasies. It is why for me that I view extra men as the equivilent of a marital aid.
The danger is a wife's desire and fantasies are not built by the same experiences. In my case, it is from seeing and reading about larger men fulfilling women when I was an early teenager and in some ways fixiating on that. Women tend to have the same type of experience (a little later) regarding their body images. Feelings of inadequacy, etc. However, with women, it tends not to be so sexually driven. They did not grow up associating their "inadequacies" with sexual performance. They tend to be two separate issues in a woman's mind.
The ability for me to more easily separate sex parts from people allows me to compromise with a surrogate to fulfill a fantasy. For my wife, not so much.
If from reading this you are under the impression I am really comfortable with my size, I still am not. I stiil often have feelings of inadequacy. However, I have learned to separate my size from my performance. But it is a learned response.