A musicologist writes: http://joebennett.net/2014/02/01/did-robin-thicke-steal-a-song-from-marvin-gaye/
Essentially, despite different chords, underlying rhythm, basslines, key, vocal melody, lyrics, they sound a bit like each other and have highish pitched male vocal.
The concluding sentence is interesting: "The act of putting an electric piano together with a cowbell and a 120BPM disco beat would need to have been judged a creative act in itself, making instrumentation and possibly even genre into protectable Intellectual Property. Which would have had massive implications for future creators of music".