Although Witherspoon once said she was “flabbergasted by how many legitimate actresses” take off their clothes, she ended up breaking her no-nudity clause for the role of Cheryl Strayed in Wild (2014). In order for her to take her clothes off, all of the parts involved must be perfect and she has to fully trust everyone involved.
Blake Lively
Blake Lively has been in movies with nudity but has never gone nude herself, she finds it very distracting! “If I am watching a scene and someone has their boobs out, then that’s all I’m looking at—I can’t help it,” she said. Lively told the Daily Mail that she'd rather her audiences focus on her performance rather than her naked body. “I just don’t think that will ever be right for me.”
Lively is a bit of a sex symbol herself, despite never appearing fully nude. Her husband, Ryan Reynolds, has appeared somewhat nude in films, most notably in The Proposal with Sandra Bullock. He probably won't go full-frontal, ever.
Julia Roberts
Yes, Julia Roberts has never laid it out for the world to see. Roberts used a body double in Pretty Woman. “To act with my clothes on is a performance,” Roberts famously said in an interview with Marie Claire. “To act with my clothes off is a documentary.” All in all: it's just not for her!