Today, I checked a thread about a custom Scoobydoo pinball machine, (imagine, something as innocent as that) and half way through it, there was a rule 34 image of Velma (not bad, but starting to get racy) and then it was soon followed with a image of a dvd porn title based on Scoobydoo.
I think the best thing to do is not post this kind of thing at all in the first place. I for one can't keep tabs on all my old posts and neither can the guy who originally started that topic to flag it as NSFW.
If you really require to prove how straight you are, there are other places like xtube for all that kind of stuff. I feel, with the wide dynamic of age ranges that could be watching, it isn't appropriate.
I think it would be nice to extend a common courtesy to keep your replies on topic to what the thread is about. Don't clutter threads with naughty images, regardless if the intention is to joke around or to spice up a thread.
I for one enjoy a good joke now and then, but it really is getting out of hand.