Season 1 is fairly clean but seasons 2, 3 and 4 have increasing sexual content.
Season 1 is completely free from nudity and there are very few sexual references.
Season 2 has a few more instances: includes scenes in an art school class, with a fully nude model, her entire back back is shown including her buttocks and her entire side including the side of her breast is shown.
Season 3 has increasing sex and nudity included: Diana's lesbian relationship we see two women kissing, in another scene a woman is shown with her shirt unbuttoned and her bra is visible. Peter's undercover work with a "black widow" she drops her dress on camera revealing her lingerie. Another scene shows a bachelor party with women dancing and lying around in lingerie. Neal and Sarah's sexual relationship. A woman is shown presumably naked but under the sheets, we then see her again in a man's shirt. Classic Greek nude statues, and another main character's sexual relationship with a girl he just met. They are scene in bed covered up by the sheets. Explicit footage in a strip club that lasts for quite some time, dancers are shown in lingerie dancing sexually. The camera focuses on a woman's breasts, a woman dances in lingerie that reveals the bottom of her buttocks, a woman opens her legs in lingerie towards the camera. The scene ends with two main characters' sexual relationship. The strip club is scene later on in the show with the same lingerie clad dancers shown.
Season 4 again increases the sex and nudity: In-depth lesbian relationship which shows two women making out multiple times. Neal's drawing of a naked woman in the background throughout the first part of the episode. Several photos of strippers in sexual positions. A couple poses for sexual pictures the man is seen with her shirt off and a woman with her dress open revealing her bra. A brief flashback of Neal and Alex in bed.