Protecting virgin eyes since the creation of the conscience!
There are some deeply religious, anti-card-games parents. According to them, Yu-Gi-Oh!—along with Magic the Gathering, Pokémon, and Dungeons and Dragons—is a tool of Satan, a portal through which innocent children shall be lured into becoming practitioners of demon magic. Meanwhile, their less-than-concerned offspring wonder how they will get their parents to let them watch the show and play the card game.
These adults are the most extreme Guardians. Most Moral Guardians just don’t want their kids to be exposed to TV that (supposedly) contains excessive violence and/or sexuality. Such parents might be belittled as "'interfering housewives'" or prudish “mother bears” by bitter artists who dislike having to reign their creative powers (Kinsella 152). However, such parents and other adult authorities have the power to frighten entertainment companies into becoming Bowlderizing Brutes, whose editing staff scramble to censor the slightest hint of Naughty and Nasty. When in packs, "Moral Guardians" often become Media Watchdogs, whole groups dedicated to maintaining the good of the young public. At their greatest power, they can spur on whole new censorship laws.
Compared to other anime—like Pokemon--Yu-Gi-Oh! has received very little tiff from concerned Moral Guardians. There's hardly been a controversy worth noting. Usually, most hubbub comes from parents asking other conservative, like-minded "media authorities" about whether Yu-Gi-Oh! is good for their kids. Some Bible-thumping may occur, but so far there's been no major riots or boycotts.