No worries - I didn't take it that way. I was just trying to say there is no 100% answer to that question. For every definition, we can come up with exceptions that people still would generally agree are a sports car invalidating the suggested definition. I've been down that road before in other forums - and it's not pretty
Well.. the true rally cars are - not the street cars

They still are performance variants of a sedan. Just like a M3 is.
Sharing PLATFORM - I don't have an issue with and I don't really consider it a huge factor. Why? Part of that is that it simply is a reality of mass production and modern car design. Designers start with a known platform because it addresses so many engineering and design requirements immediately - and then work from there.
I think the platform thing gets taken a bit too extreme when people argue 'well its built on the same platform as XYZ' - in most cases the common platform is the START point and is modified from there. It's not like both cars have identical chassis in most cases.
Platform sharing is not the same as modifying an existing car design and taking new directions (such as the Neon to SRT-4, or M3 designs from the 3 series).
I look at the purpose built argument in regards to when making decisions and compromises - does sport/performance dominate the decisions? If looking at the rear of the car.. Is it trunk capacity, or is it weight distribution that is more important? etc Is the car designed for the get-go to be a sports car, or was it a product modified to be more sporty?
A simple example... A camero is a sports car. A Z34 Lumina is not