When I first read that story, I thought that surely there were other factors involved; the MPAA site shows the rating for "some drug content and sexuality." Well, when I finally saw the film at an advance screening, I kept an eye and an ear out for any objectionable content -- and it just isn't there, in my opinion. The sex scenes are handled discreetly, very few profanities are used, and, as a point of fact, there are consequences for smoking pot, which I won't detail to avoid spoilers. Evidently the consequences weren't sufficiently severe to avoid the more restrictive rating. Either that or the MPAA is deathly afraid that any discussion of sexuality, especially among women of a certain age, should not be heard by children at the movies.
Rather than rant and rave myself, I'll just point you to Scott Weinberg's recent, well-reasoned critique, and add "pot smoking with no bad consequences" as something children shouldn't see by themselves -- as opposed, say, to the bloodless carnage perpetrated against 10-foot tall blue aliens. It's Complicated opens on Friday, December 25.