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In many episodes of the first season, Liz Lemon shows deep cleavage and, in one instance, even took off the bra she was wearing on-screen without removing her shirt.
Frequent references to sexuality and pornography, typically involving Tracy and Frank.
Season two: a sub-plot of Tracy creating a pornographic video game, which is never shown (though we do see some characters recording sexual dialogue for it).
Tracy avidly visits strip clubs, where we see scantily clad women, though no actual nudity.
Jenna typically uses her sexuality to her benefit, and she eventually acquires a boyfriend who dresses up as her.
Liz's sex life (or lack thereof) is discussed from time to time. Season five: she describes an incident where she was caught with a poster of a man on top of her and her panties around her ankles, which has led her to be scared of sex.
Season five: a magazine picture of a woman holding a leash that a man is connected to. Not explicit, but BDSM is implied. This same couple briefly makes out later in the episode.
Season seven: a woman briefly flashes, but her chest is blurred out.