I feel that pride and ego are a natural emotional shield and can be compared to bondage and nudity.
Mobility gives one a means to defend oneself. Being bound eliminates the option of defense. Clothes are a shield from touch and sight of others. Being naked eliminates that shield. Losing one's pride (through humiliation) strikes down an emotional shield.
In summary, the feeling of having one's defenses stripped from them and being completely vulnerable is a turn-on that is common to all three examples.