No it isn't off-topic since this small thread of a discussion was about the differences in ratings between the two movies. I, however did not fixate on the male nudity, however I found it funny be it you find it immature or not. I don't really care. I somewhat enjoyed the movie without having read the graphic novel beforehand.
Seeing as I am no longer 13 I could care less whether the movie is PG-13 or R since I base my interest in a movie not on its rating but whether I care to see it or not.