I totally get what you are saying and agree that there is often a double standard when it comes to "violence is okay, but no nudity, blah, blah, blah". I think for me personally the line has something to do with the "unneccesary" aspect of it that we find in media these days. And, I'm speaking of both sexuality/sensuality and violence here.
Realism has it's place and as our friend waynerbbx said, "the less-is-more approach is more relevant than we might think". The point can be made without graphic visuals or language. Sticking with Kingsmen since it is the movie we are discussing, the princess stuff at the end was totaly over the top and "unneccesary". Likewise, the earlier scene where the agent was literally, cut in half, was also over the top and "unneccessary". For me personally, there are times when choices like this in a movie do detract from my overall enjoyment of the film and for sure put movies into a category that makes them off limits to my kids.
Guardians of the Galaxy is a great example of what I'm saying about my children. My oldest son is crazy about superheroes and Guardians is a movie that I was considering allowing him to watch when it came out on video. However, they chose to put a few "over the top" scenes in terms of language and visuals that in my opinion disqualify it from adolescent/early teen age consumption. When I see things like that I wonder why the writers/directors/producers chose to limit their audience for, what seems to me, an "unneccessary" reason.
I suppose in negation of what I said earlier, While there is an increased demand of "quality" less is more type of films, there STILL are the folks that will only go to the movies if there is nudity and excessive violence. (Case in point-- the Saw series). It would be nice if movies that are pretty much children friendly except for ONE nude scene would just restrain themselves. Full nudity has never been required to tell a story.
I guess I shouldn't be a bigot about the matter, though. It's just a movie, and at the end of the day, it's just someone else's artist vision, and they obviously make stylistic and content decisions that will maximize profit while staying true to a certain aesthetic. We just pay our ten bucks or stay home.