Plenty of sex/nude scenes are not pointless, it's all down to the story, the writing. Take the one in Terminator, for example. OK it's important from an overall plot pov that Sarah and Kyle Reese have sex because Sarah has to end up being pregnant with John Connor, but it's setup so well between them it doesn't feel remotely out of place. They really need each other right then, the tone is right. And this is in an action film!
The best sex scene ever, of course, is in Team America: World Police. Check it out!