There's one near the start of 8 1/2 Women with John Standing's character arriving at the cemetery after his wife dies and being chastized for wearing white. He begins stripping down, and demands clothing from the other men, including underwear- one man is wearing black underwear, because he planned to go swimming after the service, which bugs the Standing character. Full frontal nudity.
Later in the movie there's a scene with him and the man who plays his son, standing side by side naked in a Pachinko parlor, full frontal. An interesting story, although it does have some less appealing moments.