Ekundu Cyclops. Let's hope the stuff hanging between his legs is just some sort of tail. Also this card features an Ogress sitting on some kind of tree with a surprisingly phallic shape. Get a more detailed analysis here.
Robert Bliss went through a phase of sneaking phallic imagery into his art. Goblin Soothsayer has a similar composition, and then there's Binding Agony...
On the same note, Witch-Maw Nephilim. Its appearance is rather phallic, its flavor text mentions it "making way for its passage", and its abilities (which allow it to grow until it can bypass some of the opponent's defenses) could also have a somewhat suggestive interpretation.
An article about four aforementioned secretly somewhat offensive cards.
In the art of the Rod of Spanking card in the Unhinged card set, someone is spanking a monkey.
They allowed more crap through in Unhinged than they would in a normal set. Still, there are a few standouts, such as:
The Assfolk in general, but Assquatch and City of Ass are trying to get as much crap past the radar as possible on single card.
The art on Reversal of Fortune is pretty revealing. Probably more so when you realize that the Vulshok of Mirrodin have metal that grows organically from their bodies, so she's probably not actually wearing a bra. And that's nothing—check out Alluring Scent.
Tunnel Ignus Yes, that's its name. But if the name refers to love, then the card itself is pure hate for decks that focus on fetching a lot of lands.
The various descriptions of the Rakdos manage to imply without ever actually raising the topic that among their violent, binge-drinking, gluttonous revels, there's also a ton of freaky-deaky sex going on. They're hedonists, after all. For example: Deviant Glee.
In the same way, Theros satyrs are implied to be into quite the sexual depravity. Many of Xenagos' quotes are worded in Double Entendre manners, such as his description of Phenax in the Born of the Gods trailer.
Vraska the Unseen's art◊ for the Jace vs. Vraska duel decks. As befitting a gorgon, Vraska is surrounded by "statues" heavily implied to be Taken for Granite... until you realize they're all naked men, and she's caressing them. Either Vraska turned on her lovers, or she went hunting naked men (or worse, forcefully stripping them) to turn to stone for her own amusement. Plus, the upright statue appears to have a penis.
Nightsoil Kami. It seems like an innocent-enough card... until you realize that "nightsoil" is an old-fashioned euphemism for "human feces." It's literally made of crap.
The original Legend Gwendolyn di Corsi is a temptress. She is wearing very revealing clothes and she is about to do something very interesting to that man in front of her...
The short story on the website "The Veil of Deceit" about Liliana Vess mentions sex, Liliana sees an old ogre city and sees statues of ogres doing many things including having sex. Liliana even says "I didn't need to see that."