...it would give Coppermine a certain image if it had an adult-content-specific setting.
Then how about a mature flag? Or a violence tag? Or an R tag? This is an issue that needs to be dealt with. I look at it this way - Users who want to host pornographic material with coppermine do so. And you have no issues with it. Users who want to host non-adult material on coppermine do so, and you have no issue with it. However, within certain community websites, there exists a need to have adult content alongside "G" content. And yet, that poses an issue, because even though users are free to use coppermine for adult websites, giving an option to filter it out would give you a certain "image"?
I run a graphics community website. Some users prefer to draw buildings, some creatures, and some.......the human figure. Nudity comes up. I want my site to be work/family safe, and my users do too. I have no convenient way to filter my images, short of giving each and every user who manually tells me that they are 18+ access to the "adult" sections of the gallery.
You've already stated that you're aware of users using coppermine for adult websites - And I highly doubt someone looking to use coppermine for "adult" images would need a feature to filter them out. This is a feature that is needed, and ironically it's not needed for users that want to host "adult" websites

It's for those that run communities and want to keep them open and safe for everyone interested in said community. Now that I think about it, I see no use for this feature for those that wish to use your gallery to host pornographic material.
Video games give rating for parents to filter them by content, movies give a rating so that we can filter content, and even everyday shows on TV give a rating so we can filter them for content. They're not supporting said content, they're acknowledging the content. And yet, you feel by acknowledging mature content you'd be given a certain "image"? C'maaaaan. This is a feature that's been asked for countless times by many users, and it's uses go far, far beyond the "pornographic" industry. As I've stated, I don't even see a use for this feature within the pornographic business. This is for the community runners like me and the above user, who may only have a few images that while they may belong, just don't belong to the general public.
However, in my oppinion, it should not be an "Adult" or "Mature" or "18+" flag or anything, and I'm feeling that that may be why you're shying away from this feature. In my oppinion, this should be implemented as a "Flag Settings" feature. Implement flags on the back end and give the admin an option to use custom flags. Lets say for my gallery, I want to have a "Nudity" flag, a "Violence" flag and a "XXX" Flag. To add these, I just go to the backend of the gallery, type these into the "custom tags" field, hit apply, and then when a user uploads an image they have radio buttons for said flags that I have set that they can check prior to uploading.
It's a complex feature, but there's no denying that it's needed. And if implemented the way I've described ( which may be hard to understand, I admit ), it frees coppermine from getting a certain "image" that some people seem to fear. A filter system with custom flags that can be added by the admin for the user to select prior to uploading an image, would, in my oppinion, be an amazing feature for coppermine, and I'm sure the community would agree.