Of all the big Hollywood stars, think of those who actually disclosed their integral nudity in film: extremely few! And, if you examine the context, they only did so when their once bright career was at a desperate stall.
I can think of Sylvester Stallone in Demolition Man, Maria Bello in A History of Violence, Jamie Lee Curtis in Trading Places, Kate Winslet in Titanic, Halle Barry in Swordfish, Eva Green in The Dreamers, do I need to mention Sharon Stone in Basic Instinct? The most daring scene of integral nudity and sex together, however, was to me Charlotte Gainsbourg's in The Antichrist, where one of her nude scenes reached pornographic heights.
There are, I think, two main reasons why actors might decide to do nude scenes: they either need to boost their career or they need to climb out of a ditch of oblivion. In my opinion, the latter is a dirty trick but it sometimes works.
The former is absolute insanity.
Yet, actors actually do give the impression they are totally undressed during sex scenes. Are they?
Most of the time, no.
Keep in mind that actors can be shy too. If they felt so comfortable being in genital contact with each other on camera, they would probably do porn.
What they do is they might ask the make-up artist to help them protect certain areas of the body with skin-like and similar types of protective layers.
Remember, they're just acting! For very revealing and hard-core scenes, instead, they are usually replaced by actual porn actors.
They might not show themselves naked in film, but are they really naked on the film set?
Keep in mind that, nowadays, everybody has a cell phone, with the added possibility of taking digital pictures very quickly and at anytime, film crew members included. No serious actor would want to expose themselves that way, especially in the intimacy of about 70-80 people moving like ants around them during filming.
I hope to have briefly satisfied someone's curiosity over such delicate and highly choreographed scenes.