There's a member of the Lonely Hearts Club Gang who wears a broken noose around his neck. Given all of them depressed and angry for being dumped by their girlfriends, he probably tried to kill himself.
In "The Great Unwashed", Tiff calls Jenny a "circuit jerk", which sounds extremely similar to the rather profane term "circle jerk".
One of the recurring antagonists is a gang of alien biker chicks. Most of them are dressed like dominatrices, especially the leaders, who drags around a smaller, apparently male member of their species by a leash.
One episode reveals that this is a custom with their species as husbands are to be treated like domestic slaves or house pets, which is what would've happened to Brad after he married Tammy.
During his explanation of puberty, Brad inexplicably shows Jenny a 1950's style showreel of children growing into adults. Little Billy grows up into a handsome man named William, while Little Sally's adult form - a Hot Scientist with an hourglass figure - is dubbed by Brad as "...uh, Big Sally."
Embarrassed by robo-acne, Jenny puts a bag over her head and complains "it smells like fish tacos!".
In one episode, Jenny is expelled and has to be home-schooled. Dr Wakeman sets up a fake prom, and pairs Jenny up with XJ-8, her sister.
Escape from Cluster Prime:
Ms. Wakemen takes off her coat. It cuts to a between-legs angle where we see Tuck and Brad looking away, but Sheldon's eyes wide open. Then it switches views to see that Wakeman is in her Skyway Patrol uniform.
When the Cluster starts the second wave of its invasion, Brad watches in horror, and the camera pans down to his legs. Then Tuck drops down from behind him with a "glop" sound effect, to give the impression of Bowel-Breaking Bricks.
In one episode she has to keep saving the world hidden from a guy she likes, and since she already used having to go to the bathroom as an excuse, she leaves a blow-up doll of herself in her chair. She comes back and lets the balloon blow off—and it zooms into Sheldon's desk, who closes it with a smile and questionable intent.
The beginning of the episode "Good Old Sheldon" has Jenny just finishing telling Brad and Tuck how (she thinks) babies are made, leaving them with horrified expressions.
In "Voyage to the Planet of the Bikers", the Space Bikers are revealed to have normal day jobs on their home world as staff and faculty in an elementary school. Tammy is a teacher shown standing in front of a blackboard with a diagram reading "birds + bees = babies". While pointing to this diagram she directs her students to turn to page 69 in their textbooks.