We we're actually planning a more extensive post on this very question tomorrow, but Slate beat us to it, and did a very good job at it too:
To steal a phrase from a colleague, Love Is Strange is, in comparison, as clean as a Mr. Rogers episode. Yes, there are a few stray curse words, but nothing that warrants more than a PG-13. Truly R-worthy offences are otherwise absent: There is no nudity, no sex, no skull-bashing or violence of any kind—unless you consider the (unfortunately realistic) situation of gay couple being forced out of job and home for registering their relationship with the state to be a form of violence.
So we'll direct you over to them for further reading, and note we second every single thing they say. The MPAA is being absolutely ridiculous with this judgement, and we hope you go see 'Love is Strange' this weekend despite it. And if you're not of age -- get yourself a fake ID (and if you're a theater owner -- we strongly encourage you to look the other way).