As far as vampires and werewolves go, we have Vampires as a subcategory in Originals. In the fandom archives, it's usually pretty obvious which fandoms are prone to having vampires and/or weres, and requiring authors to tag for the obvious when writing a True Blood or Teen Wolf fanfic seems excessively burdensome on the authors. We also need to keep in mind that tags are contained in the Summary Field, and we have a 240 character restriction in that field. We'd like to leave authors room for a summary in addition to those tags.
We try to keep tagging to those things that we believe readers need to be forewarned about in terms of content. We add tags as required for content of that nature, like the recent additions of AB/DL, TB/DL, Ageplay and Humanoids. That is content some readers may find upsetting, or objectionable, and therefore we deem it taggable. If we get a huge surge of readers complaining vociferously in email about marriages and weddings, and/or cheating and infidelity, we'll of course look at adding those tags. But for now, I haven't seen any complaints about that content.