Tumblr, a microblogging service that’s impact on internet culture has been massive and unique, is preparing for a massive change that’s sure to upset many of its millions of users.

On December 17, Tumblr will be banning porn, errr “adult content,” from its site and encouraging users to flag that content for removal. Existing adult content will be set to a “private mode” viewable only to the original poster.

What does “adult content” even mean? Well, according to Tumblr, the ban means the removal of any media that depicts “real-life human genitals or female-presenting nipples, and any content—including photos, videos, GIFs and illustrations—that depicts sex acts.”

This is a lot more complicated than just deleting some hardcore porn from the site; over the past several years Tumblr has become a hub for communities and artists with more adult themes. This has largely been born out of the fact that adult content has been disallowed from other multimedia-focused social platforms. There are bans on nudity and sexual content on Instagram and Facebook, though Twitter has more relaxed standards.

Why now? The Tumblr app was removed from the iOS app store several weeks ago due to an issue with its content filtering that led the company to issue a statement. “We’re committed to helping build a safe online environment for all users, and we have a zero tolerance policy when it comes to media featuring child sexual exploitation and abuse,” the company had detailed. “We’re continuously assessing further steps we can take to improve and there is no higher priority for our team.”

We’ve reached out to Tumblr for further comment.

Update: In a blog post titled “A better, more positive Tumblr,” the company’s CEO Jeff D’Onofrio minimized claims that the content ban was related to recent issues surrounding child porn, and is instead intended to make the platform one “where more people feel comfortable expressing themselves.” “As Tumblr continues to grow and evolve, and our understanding of our impact on our world becomes clearer, we have a responsibility to consider that impact across different age groups, demographics, cultures, and mindsets,” the post reads. “Bottom line: There are no shortage of sites on the internet that feature adult content. We will leave it to them and focus our efforts on creating the most welcoming environment possible for our community.”

The imminent “adult content” ban will not apply to media connected with breastfeeding, birth or more general “health-related situations” like surgery, according to the company.

Tumblr is attempting to make aims to minimize the impact on the site’s artistic community as well, but this level of nuance is going to be incredibly difficult for them to enforce uniformly and will more than likely lead to a lot of frustrated users being told that their content does not qualify as “art.”

Tumblr is also looking to minimize impact on the more artistic storytelling, “such as erotica, nudity related to political or newsworthy speech, and nudity found in art, such as sculptures and illustrations, are also stuff that can be freely posted on Tumblr.”

I don’t know how much it needs to be reiterated that child porn is a major issue plaguing the web, but a blanket ban on adult content on a platform that has gathered so many creatives working with NSFW themes is undoubtedly going to be a pretty controversial decision for the company.