Not sure a password protected idea is necessary, if it's in the title as to what's inside the thread, then common sense tells you whether to look or not.
I don't mind porn, i find it amusing when used in context of certain posts in the ' anything goes ' section, but once or twice i've winced when realising my boys are in the same room and a huge picture appears, i laugh it off, but again it could prove embarrassing with younger children around, but it goes back to the point at the start of this post, stick a warning/subtle hint in there.
Only issue i can see is if someone posts a porn picture on someone else's thread regarding a different matter, that thread then becomes a ' NSFW ' area without the original thread makers input at all, how would you approach that scenario?
A way around that would be to have all pictures posted be confirmed by a mod before being allowed to be viewed in the post, but that's going to increase an awful lot of work with alot of non porn related pictures.
Another simpler idea is to remove the direct posting of images, allow only attachments to be used, a small thumbnail like alot of people do already. Perhaps using a NSFW icon on the thumbnail, i know this is possible.
Overall though it's down to the person clicking the thread title or scrolling carefully when Fanatic posts (
) in the anything goes section
