Hardly a rip off. Thinking Out Loud bears no resemblance to Let's Get It On in the copyrightable bits, the lyrics and melody. These songs are simply not the same. So if you think you can sue over a chord sequence or rhythm then every song written in the last 60 years is a rip off.
This is a smash and grab. An opportunistic attempt to con some cash out of ES, hoping he doesn't want any "bad" publicity.