Naked with Marlon Wayans 2017 is a wonderful romantic and tastefully done comedy about an embarrassing moment while being naked. A modern take on Groundhog Day where the groom has to find out why he's naked in an elevator on his wedding day and get it right this time around?
Starman, I think it was, with the alien born naked from a drop of blood on the snow.
The drop of blood was for the TV show so they showed above the waist and below the knees only.
The movie was from a lock of hair, and included full rear nudity.
Similar effect was The Time Travelers Wife, where he left his clothes behind when he did his time traveling and appeared nude at his new time and location (again rear only to keep the PG-13 rating).
The most natural casual nudity I recall seeing in a movie - meaning, just a couple who had been together for a long time and were sometimes casually naked together - was a scene from the 2003 movie Open Water. A couple was on a group scuba diving excursion and in one scene the couple is in their hotel room, lounging and reading before going to sleep at night. Both are nude, but not for the purpose of sex. They're just being. There may have been a second scene where the wife was walking around naked while getting ready, but I'm not sure about that part. It's been a long time since I saw the movie.