As Malvasiah said, it's a turnoff not because of the pic itself (penis is just a part of human male body, it's natural, so nothing is wrong with it), but because being a clear sign of the person's plain and simple mindset. Usually it comes with fixation on the size of penis and other porn stereotypes, with all the weird ideas about sexuality that porn industry cultivates. In most cases, men with such avatar pics are boring and stupid. Of course, there're always some exceptions, but will you want to dig through the heap of garbage to find one?
