(CNN) -- When makers of one of the most anticipated video games of the year invited users to help design part of the game, the gamers jumped at the chance to create animated characters.
Users will have the option to incorporate other people's "Spore" designs or choose to block them all.
But some took it upon themselves to create something entirely different: a new kind of Internet porn.
It started when the makers of "Spore" released a Creature Creator program that allowed users to develop their own characters to drum up hype before the game's scheduled release in September.
The game, a joint venture from "Sims" creator Will Wright and Electronic Arts, allows users to create a unique creature and then control its evolution from a single cell into a complex cultural civilization.
Within 24 hours of the Creature Creator's release, gamers had gone creature-crazy, designing millions of critters that were all thrown into a database and shown on a YouTube channel for the public to see.
But scrolling through the database -- past the three-legged sea horse, past the seven-eyed wildebeest and the half-motorcycle-half-pig -- revealed something many users didn't expect. Buried among the more wholesome attempts were two-legged dancing testicles, a "giant breast monster" and a four-legged "phallic fornication machine," for starters. Watch Bradshaw demonstrate the "Creature Creator" and "Spore" »
These naughty -- some would say obscene -- creations have spawned an Internet meme nicknamed "Sporn," short for "Spore" porn.
For EA, the developer of "Spore," it's the downside to tapping into the booming user-generated content arena, which has made sites like YouTube, Flickr, MySpace, Facebook and Second Life so popular. These games and sites often allow people to create and host their own content in addition to creating cartoon personas, called avatars, for themselves.
Many of the popular user-generated content sites have faced similar challenges in trying to control obscene material. In Second Life, users can read Slustler, a cyberporn magazine, or buy programs that allow them to have animated sex with other characters.
"Whether it's modeling clay, dolls or crayons, a small number of people can be counted on to use it for something vulgar," said Lucy Bradshaw, "Spore's" executive producer.
Despite its "Sporn" issues, "Spore" is poised to become one of the most popular games because of its ability to let people tell their own story, rather than one mapped out for them, Bradshaw said.
"Rather than putting players in the shoes of Luke Skywalker or Frodo Baggins, we're giving them the opportunity to be George Lucas or Peter Jackson, as they create their own universe from scratch," she said.
But for every George Lucas and Peter Jackson, there's always a spoiler.
Enter the Spornmaster, a 37-year-old Web developer who refused to give his name for this interview. He, like many others, has spent hours creating characters and turning them into sexual beings.
"It came up simply as something silly and juvenile to do," he said.
The creatures are not just static. Users can create animated scenarios for the characters to engage in, some of which include sexually graphic acts.
When EA got word of the Sporn creations, it began working with YouTube to pull them down. Players who repeatedly upload "offensive content" are warned, suspended and eventually banned, Bradshaw said.
But the policing isn't restricted to EA and YouTube. Users also are able to flag and report content that they find offensive.
That angered some content creators, who feel that they should be able to create whatever they want.
"It was a totally ridiculous overreaction," the 37-year-old Web developer said. "I admit it is silly and juvenile, but I don't think there's anything perverted, vile or awful about it. If people find it offensive, they can simply not search for it online. No one is forcing anyone to see this content."
In response, he created a site to preserve as many of his naughty creations as possible. He said many other creators of Sporn have told him that they too were only joking around. But the Web developer and other Sporn creators have had their share of critics. On blogs and message boards, some have called these creators perverts.
"I consider this very similar to child pornography, at least to the extent of distributing the material to children," said 18-year-old Michael James from Calgary, Alberta. James said he has flagged about 10 of the "disgusting" creations.
EA plans to make sure nobody sees the content if they don't want to, Bradshaw said. When playing "Spore," users will be given three choices regarding people's creations: to receive no outside content, to receive content from buddies only or to receive all external content.
Bradshaw hopes the sexual characters don't spoil "Spore" for everyone or get in the way of what she says is a revolutionary game.
"User-created content gives players total control over their game experience and empowers them to express their creativity in ways that they never thought possible," Bradshaw said. "It also gives them a powerful emotional connection to the game, since they've created the world from scratch."
Miles Moffit, a gamer attending the University of Georgia who has created tons of "clean" characters on his own, is glad to know EA will be regulating what makes it into the game.
Moffit is eagerly awaiting the game's September 7 release. And if by chance a Sporn character shows up in his virtual "Spore" world, he has a plan.
"My initial reaction to discovering it in my final game would be to ban it so it wouldn't show up again and then blow it to pieces for the sheer satisfaction of it," Moffit said. "Go ahead, create a walking phallus. See how long it lasts in the databases and galaxies of 'Spore.' "