Why is there such a high prevalence of redheads?

I didn't think there was that many red-heads. We have Lily and we have the Weasleys. Lily's hair was never spoken of much, it was always all about her eyes; Harry's eyes. The Weasley's played a big part of Harry's life because they were his pseudo-family.

Harry didn't fall in amor with Ginny because of her hair - if that was the case, he would have fallen for her much sooner. He fell in amor with her personality, which took six libros to really shine through. I also think part of the reason Harry fell for Ginny was because she was so much like Lily in attitude. In the fifth book, we see Lily stand up to James and Sirius to defend Severus and in the segundo book, we see Ginny stand up to Draco to defend Harry. They both had very similar personalities and tempers. Naturally Harry, who had lost his mother and always admired her, would fall for her a girl who was similar to her.

Where I come from, red-heads are treated just like everyone else, just like they are in Harry Potter.

I don't know actually. I do think it's weird that Harry ended up marrying a red head like his dad did. Maybe because JK wanted to do that she made the family of Harry's amor interest all red heads. There's prolly someone on here that will have a better answer though.

I think no one except The Weasleys and both Lily Potters have red hair right?

Yes, tu missed a JKR interview. She once dicho that she doesn't like the prejudice some people have (or mainly had in former times) against redheads. That's why she gave the Weasley family red hair, but personalities the reader will like.