Towards the end, Philo (Clint Eastwood) says "Clyde, I might have a problem getting you this one" he looks at a playboy magazines zine picture of a naked woman (from her side, the woman is not super clear) that he got from Clyde.
There are four sexual references.
Near the begining of the movie, a man is in a street fight. A couple of girls cheer him on. When the man turns to face the girls, the camera slowly zooms in on the girls' breasts, but the focus abruptly switches when the man is bashed from behind by his opponent.
In one fairly long scene, everyone in a motel gets stricken with the urge for sex: one older man observes his wife's hips (from behind) swing back and forth. The man becomes mesmerized, and fantasizes the image of a hula girl's swinging hips in a grass skirt. One shirtless man, and towel-robed women climb into bed, and continuously kiss each other passionately. Sex is implied for both couples.
During the same scene, and elderly woman observes an elderly man peek through the curtains of the motel (watching the two couples have sex) and tells him that "it's not nice to watch people go 'humpity bumpity...'"
A little later, a man steps out of his house, and rips open his robe, flashing who he thinks is a woman (it's actually an orangutan dressed in a night gown). There is no actual nudity, as his genitials are blocked in the frontal camera shot.