The guilty pleasure anime category encompasses the shows you don't want anyone to know you're into. For some people, these are fan service anime - these series can be a ton of fun, but viewers might feel weird about them, either for moral reasons or simply because they'd be embarrassed to have someone walk in on them while they're watching. For others, a guilty pleasure show might be super cute in a way that doesn't fit their image.
There's no reason an adult can't enjoy Chi's Sweet Home, but it may be hard to explain to a non-fan why they're watching a show aimed at small children. A handful of shows just aren't well-written or are only good in parts. If someone is a literature major who regularly reads the classics, they might struggle to articulate why they're so passionate about Yu-Gi-Oh! Regardless of individual taste, there are a ton of popular shows people wouldn't necessarily boast about watching.
It wouldn't be out of line to claim High School DxD is centered around nothing but gratuitous displays of the women in the show. However, there is a plot in there somewhere: Issei Hyoudou is slain, comes back to life as Rias's servant, and ends up locked in conflict against a series of angels and demons. High School DxD may not be a work of literary genius, but it's a surefire way to have a good time.
Fans shouldn't bother trying to convince themselves they're watching Monster Musume as fantasy aficionados. For that, they could watch a much tamer series, like A Centaur's Life or Interviews With Monster Girls. No, if they're watching Monster Musume, they're there to watch Kimihito Kurusu get dog-piled by his harem of humanoid magical creatures.
There's just enough of a plot to keep things from getting boring, but Monster Musume is probably best enjoyed in the privacy of one's own home.
This is an insanely well-animated show with a unique and engaging plot. Unfortunately, that plot serves as one big vehicle to get the teenage characters undressed. There's also an outrageous scene in episode 16 that's drawn in a gratuitous way that's hard not to feel squeamish about. It's a wonderful show overall, but it has moments that make you feel guilty about loving it.
If bingeing on fan service anime begins to feel more like guilt than pleasure, Shimoneta: A Boring World Where the Concept of Dirty Jokes Doesn't Exist is here to help. This anime champions the idea that repressing desires leads to disaster.
Some of it is hilarious, like Ayame Kajou's superhero outfit, which includes underwear worn as a hat. Some parts are less funny and more cringe-inducing, like when the usually straight-laced Anna Nishikinomiya forces herself on her love interest without regard for his desires. Shimoneta is a total guilty pleasure anime, and its message is that acknowledging those pleasures is important for a healthy society.