You don't need gore to sell a movie, unless your movie is based on gore.
You don't need sex to sell a movie, unless your movies is based on sex.
Superhero movies are based on the concept of a 'superhero'. The most profitable movies of all time, have been geared to be watchable by 'families'. Now, family-friends does not mean that Mom and Dad and their 2.3 kids are all going to the theater, it also means that parents will permit thier young daughter, Barbie, go out with 'Ken' to see a movie that they feel comfortable sending young adults on a date to go and see.
There are plenty of movies that fulfill the 'R' rated audience demands. These seldom are huge cash machines, in fact one could argue that as a percentage of all movies made, the 'R' rated movies make less money than the 'G' rated movies.
As a movie executive, the goal is to make money from your movie. You can do like Pixar and make a fortune on your movie, then make double that in after-market toys, pajamas, T-shirts and lunch boxes. Why? Because their movies appeal to young and old alike. Parents buy the DVD, and they know that when the grandkids, cousins, neighborhood kids come to visit; they can watch those movies without any parent being offended.
A superhero movie's demographic is going to be minors to young adults. The older classics (Iron Man, Batman, Spiderman) are going to appeal to a wider age range than the 'Watchmen', simply because of the age of those people who read the comics. Simply stated, when you make an 'R' rated movie, you immediately remove the family and young adult (12-18) group from the customer list. Those 18-90 yr olds are going to be more mature, those who are 'adults' will want a compelling story, and there had better be a good reason why the story required them to find babysitters. Boobs, graphic violence jsut doesn't cut it. Now, movies that tell a historic event fall into a category that is unique. The 'R' rating is justified, in fact 'defended' in movies like "Saving Private Benjamin", "The Passion", "The Patriot" and "Schlinder's List" for the fact that the movie depicts actual events.
Having a big blue guy with full frontal nudity for 3 hours isn't defendable. There is no reason, other than the Director's wish to shock his audience. Guess what? Most 18-24 yr olds weren't shocked, and those 25-90 didn't find the rationale for having to include the scenes that gave it an 'R' rating defendable. Hence, the movie didn't make money.