I can't see that this tag would ever be used for any purpose other than "having another tag on it". Do we really need a sex tag? Wouldn't the fact that the question is obviously about sex be enough?
I don't necessarily disagree. However put a bit of a perspective on it I think its meant to be used when a question is about sex in movies in general, rather that just another tag alongside a movie name.
What should someone tag the following:
For the purpose of having something for people to vote on here - I would propose keeping the tag for the purpose of tagging questions that specifically relate to the use of sex in a movie or TV show.