I would define a sports car as a car that is designed for sport/speed to the detriment of other facets. Perfect examples are Noble's/Elises/VX220's etc. They are super light and all luxuries are stripped out such as carpet,AC, electric windows etc. Built for speed to the detriment of comfort.
By this definition I don't think that typical Lambo's & Ferrari's are true sports cars as they contain an element of GT about them by the very luxuries that they offer. This is evident by the fact that both Lambo and Ferrari bring out hardcore versions of their cars such as the Superleggara, Scuderia and Challenge Stradale etc etc.
I think that all cars are on a sliding scale and most contain certain elements from one, two or even three categories. A 350z is definitely a GT due to its weight etc. I couldn't ever view a car like an M3 as a sports car as it has 3 rear seats and a massive boot......