Of my previous gf's who have cuckolded me, the feelings have always been the same. The only way I can describe it is to compare it to that feeling I had when I did almost any sexual act the first time. Pure thrill, sensual turn on, butterflies in my stomach, and often the feeling of being bad feels good thrown on top.
When I would watch them with other men it was incredibly erotic and in a way that was also emotionally painful, the jealous thoughts themselves are parts of the turn on, like 'I can't believe she's letting another man inside her in front of me!' while I'm just staring, gaping at them, I have other thoughts like "He's fucking her better than I do", and "I can't believe he just came inside my girlfriend, right in front of me, omg".. and each thought tweaks my sensations pulsing literally through my nipples down to my scrotum.
I also feel the humiliation and shame of it. The say people who like to be spanked and abused are sadists, that's physical sadism. Cuckolds are more like emotional sadists. We enjoy the physical and sensual combinations. Chastity cuckolding is a good example of that. Ruined orgasms, denial, etc.. All make me feel that way.