As far as I know, our religion doesn't give us a license to sin just because one is adult. We are not like atheists who believe that everything is ok to watch/read for an upper eighteen person.
  If so, why the phrases like "warning, delicate topic" or "warning graphic/adult language" are used before some  articles as though being warned would make it moral?

Also, inappropriate for young children, general discussion with older ones and parents shuold not discuss their sex activities at all with children...I for one do not wantto know what happened in the fall of 1970....suffice to know, my parents conceived me....

The warning "adult" applied to sinful material is a sick joke. There is nothing adult about it (it's juvenile if it's nothing else), and it is definitely sinful.

"Graphic content" could just mean that, for instance, in a tv news story, there are going to be images of bƖσσdshɛd you might not want your average child to see, but as a news story, well... that's what's going on. It's not sinful to look at the bƖσσdshɛd, to my knowledge, (unless you suffer from some bizarre depravity that makes it sinful for you) though it probably isn't good for anyone, but that is one of the few cases in which such a warning does not always cover up things that NOBODY should be seeing.

It's really sad to see a warning like that, as I just did recently, in a book supposedly written by a Catholic, followed by things nobody should have ever laid eyes on, but... sadly, there are some poor individuals who have lied to and convinced themselves that by putting that warning, they've somehow made it "all right" to have that content there. A grave misconception, to say the least. Tragically, this sinful kind of content will have very real consequences... it will corrupt minds, hearts and souls... no matter HOW old they are.

There is never an excuse for anything that will lead other (normal) people into sin, because it is a sin to lead others into sin. Your instinct that there is something wrong or fishy here, is right on!