Actor who stripped down: Pretty much everyone who isn't an A-List actor
Why they're fully nude: Women are but mere objects in Stanley Kubrick's final film.
Like Salò before it, the bulk of the female characters in Stanley Kubrick's final film exist for the single purpose of pleasuring the deep-pocketed and powerful men who surround them. They are bodies, there for the taking—ready or not.
Based on Arthur Schnitzler's 1926 novella Dream Story, it takes but just one minute for a bare ass (Nicole Kidman's at that) to make an appearance in Eyes Wide Shut, and less than 15 minutes for the first instance of full-on nakedness (this one following a drug overdose) to happen.
Overdose would be an appropriate word. From there on out, the female form is on full display—standing, sitting, walking, kneeling, and definitely in flagrante—throughout much of the remaining 144 minutes. So much so that you almost forget it's there after a while. Kind of like wallpaper.