There is ZERO consistency in how lines are drawn. The 1999 South Park movie was, on some level, a meta-commentary on just how vile a movie had to be to get an NC-17 rating without including any graphic sex. You're probably doing better than the MPAA. :-)

I think HTML is a gift for spoilery warnings--lets you set some parts of your header to black-on-black or white-on-white so that it's up to the readers--which goes back to the caveat-lector philosophy, that you're taking responsibility for reading something without looking at the header first.