Thats a personal decision. If you feel guilty listening to secular music, then dont listen to it.
A lot of people will argue that there is nothing wong with reading love poems, so Why would it be wrong to listen to love songs.
Me, I listen to a lot of secular music, and it has helped me with my playing. Not all secular artist are vulgar and nasty. Stevie happens to be one of the clean ones, and If you study his music, it just might help you with your creativity. Face it, in order to play today's gospel musc, you have to know Jazz, Latin, R&B as well as other styles of music. If the words in stevie wonder's offend you then dont listen to him.