I agree with most of your post, but I'd hardly put the Widow poster in the same category as the Mystique one. Widow is fully clothed, Mystique is portrayed as naked despite her strategically-placed scales. That character has been unnecessarily and deliberately portrayed in an overly-sexualized manner in every film, ridiculously so. In the comics Mystique always wore clothing like anyone else, but Singer & co. decided to strip her down and make a skin show of her.
There wasn't any pompous pseudo-feminist outcry about Mystique's nearly naked look when the Empire covers debuted the other day. I wonder why there's such a difference between how that character is perceived and the reaction to Natasha. Someone in this thread brought up Katniss, but why not mention JLaw's other role? Because it wouldn't support the nonsensical argument?