Blue Valentine has been lauded for its visceral storytelling of the love that's dissipated between a married couple. But what heightens the realism the film strives for is the use of Spanx in the sex scenes between Williams and Gosling.
The movie is about a couple who gives one last try at a cheesy motel to save their failing marriage, infused with romantic flashbacks of their courtship, that includes some pretty racy sex scenes that initially warranted an NC-17 rating. It's unclear if the Spanx were a character choice, but given how Method the director and actors went for this project, it can be assumed that they were.
Personally, I think it's great, and perhaps one of the "realest" aspects of Blue Valentine—certainly more real than a once-hip couple who, after only six years, devolved into a hell of fatigue, bitterness, male-patterned baldness, dorky dad glasses, and a minivan. (They had one kid. What's wrong with a sedan?)