The definition of "cuckold" to is a bit fuzzy still.
A few years ago, I expressed to my girlfriend (now my fiance) that I had thought about wanting to see her fuck another man. At first she wasn't into it, but accepted and respected that it was my fantasy. Later on, when the right opportunity presented itself, she asked me if it was still something I wanted to see. It was. She had a male co-worker that she was attracted to that was moving across the country. She arranged it so that, the weekend before we left, they hooked up and I came along to watch. It was exciting and fun and it became another part of our sex life after that. She had a great time being intimate with men she found attractive and I saw it as a really fun, hot show. No jealousy or weird feelings every developed because of it. On top of that, being a witness to seeing her lose all of her inhibitions allowed me to notice things I've never pick up on while we were having sex. You notice little things that you might miss while focusing on trying to pleasure somebody directly.
It was never about humiliation or eating cum or anything like that. I just liked to sit back and watch her have a great time. Sex is beautiful and amazing and as great as our sex life was and is, there's something awesome about getting to see her enjoy herself without being directly involved every now and then. I don't consider that "cuckholding," I just consider it voyeurism, but I guess everybody has their own definitions.
I've realized that it is about humiliation for some. And, with others, I think there is a gay or bisexual element to it. But even though I don't "get" that or have any desire to be involved with that kind of thing, I don't see anything wrong with it. If it's safe and between consenting adults and nobody is being manipulated into doing it, so what?