>>15209546This may be permissible under the assumption that anyone can be expected to have more or less the same preferences and will have prior knowledge with regards to what kind of a site they are going into, but when we consider that many people who come into /bant/ (or /pol/), as opposed to boards exclusively dedicated to pornographic image posting, such as /e/ and /h/, might not necessarily have either the intention or the necessary precaution to either avert their eyes or to be willing to look at lewd images.
Under Kant's categorical principle, one must act not in accordance to what one knows or expects that other people might do, but rather to do what any person, in the absence of certain knowledge about every single detail of their circumstances, would be necessarily forced by their own consciousness to do.
Therefore, ignoring altogether the issue of whether onanism is itself moral, I would argue that instead posting lewd images in a space not meant for it could be said to be immoral due to it not respecting the sensibilities and tastes of other people.