If Femdom is the redefinition of the sexual hierarchy (women rule / men serve), CFNM is the physical representation of that redefinition. There is no doubt that nudity makes a man more vulnerable, especially if the woman or women are fully dressed. But it goes much deeper than that.
Take the picture above: it is probably some kind of party and a naked man is serving the amused young woman some wine.
His nudity and her being dressed instantly establishes a D/s relationship. It redefines the man as a sex object which is traditionally how women are treated (think Hooters or “Gentleman’s Clubs”).
In addition, he cannot hide the fact that his bright red ass shows he has recently received a spanking or paddling; for some misbehavior. This reinforces the sexual humiliation of his woman’s punishment beyond the immediate ceremony.
He also cannot hide his erection, or his small shriveled manhood, or maybe it is locked up in a male chastity device…
There are many other aspects of CFNM, but they all share some aspect of the naked man’s vulnerability, humiliation and being put in his place as the clothed woman’s (or women’s) inferior and/or sex object.