I understand that you are uneasy with the two songs sharing similar instrumentation and tonal imagery. However, Robin Thicke is legally permitted to use the same drum set, the same bass guitar, the same piano keyboard, the same cowbell, the same mixing board, the same effects, the same singers and any other tools that Marvin Gaye used in his song just as long as he (Thicke) is able to produce a song or composition with it's own unique lyric, melody and musical arrangement. In this case, different lyrics + different melodies + different musical arrangements = different songs.

Who can patent pitch, rhythm, harmony, timbre and tone? These elements were in existence and in use before any of us or our musical idols were born.

Allow me to use a non-musical example. Two separate cooks are allowed to bake from scratch, yellow cakes with chocolate icing that look and taste similar. They are also allowed to package them and sell them to the public as long as they do not share exact ingredients and identical packaging or anything that might blur the lines of difference between each cake. Where the two cakes will differ is in the balance of ingredients used in their creation. Both cakes will probably contain sugar, flour, butter, chocolate and eggs but the quantities of each ingredient will vary between the two. This means that the two cakes will wind up sharing the same taste and appearance while not being "exactly" the same.

So it is with the two songs in question. Like the cakes mentioned above, they both contain the same ingredients in their mixes so they both have the same "flavor". This still doesn't mean that they are the same song because the balance of ingredients between the two is noticeably different.

I hope this helps some.