Sexual politics is discussed in almost every episode, and implied sexual activity takes place in them (sometimes stronger implications than others). In the first episode, Jess strips naked for her boyfriend, holding a large pillow in front of her "swimsuit area" as she dances. We see her shoulders and legs, and when she turns around, surprised that her boyfriend is cheating on her, a bow drops from between her legs.
Season one, episode three ("Wedding"): Schmidt is worried that a woman he always hooks up with at weddings will have sex with him again. Flashbacks show them making out in closets (clothed). At the end, she is sitting on top of Schmidt in bed (clothed), and he appears to be naked (his crotch covered by her).
Season one, episode four ("Naked"): Jess walks in on Nick naked, (we don't see below the waist), and she tries to make it up to him by letting him see her naked (we see her shoulders). Throughout this episode, Nick constantly hits on a girl, and is twice seen passionately making out with her and disrobing. The second time, she strips all the way to her underwear (sex isn't implied either time, as they are interrupted).
Season one, episode eight ("Bad in Bed"). Jess buys "naughty" lingerie, and wears it (with a bra and panties on) when she tries the second time to have sex with Paul. Both times, they're seen kissing, caressing, stripping each other of clothing and lying in bed. The second time, they make crass remarks to each other, and Jess slaps Paul on the backside to excite him. Both times, they don't have sex, but the next day, the passionately make out in an elevator as the scene cuts. Sex implied.
In the last few episodes of season one, there's a running physical gag in which someone's groin hurts whenever they become sexually aroused. No nudity and played for laughs, but some non-explicit sexual references are spoken (once as a prank) for the purpose of arousing the man.
There are crass sexual remarks in basically every episode, and Cece (model) is seen a few times in underwear.