Valve is taking aim at adult content on Steam, and has threatened to delist a number of games it had already approved unless they're amended to remove sexual content.

A growing number of affected studios and developers have been sounding the alarm on Twitter, and seem taken aback at the abrupt nature of the decision.

Lupiesoft, an independent studio that creates visual novels containing adult content, says Valve has threatened to pull down its portfolio in the next two weeks after hearing "reports of pornographic content."

Notably, the studio claims to have followed Steam's guidelines to the letter, and even says its publisher, MangaGamer, was told its games were safe on Steam after meeting with a Valve spokesperson.

"It seems this new change violates what Valve agreed to before, and the entirely of the visual novel genre of Steam is feeling the effects of this nuclear option," the studio wrote on Twitter. "Every developer big and small, no matter how much they followed Steam's guidelines, is having games pulled.

"This puts us in a bad situation. while funding for Dizzy Hearts (the studio's next project) is secure, Mutiny!! was our biggest title. This threatens our future when a platform like Steam can decide on a whim to just pull our games even when we follow their guidelines."

MangaGamer offered more insight on its blog, and explained they are "wholly in the dark" following Valve's sudden change of heart.

The publisher revealed Valve failed to provide any clear guidelines or standards before threatening to pull its games from Steam, and reiterated that it went "to great pains" to run the game's content by Valve representatives, including sending along "every potentially questionable graphical asset" as well as advanced builds.

"We stand by our assertion then that content of the game is not pornographic, but an earnest and tasteful exploration of sexuality," wrote the publisher.

"Despite our best efforts to respect both the letter and spirit of Valve’s content guidelines as we were made to understand them, we are wholly in the dark with this latest apparent shift in policy as no clear guidelines or standards have been provided."

Other developers including HunieDev and Neko Works, which also create titles featuring risque content, have been hit with the same email, with Valve again targeting those creators for inserting "pornographic" content into their games.

We've reached out to Valve for more information on the purported policy changes.