Yes, he's a fictional character of a fictional book. But most people realize that when an author writes a book, or when a producer films a TV show, etc, he has certain values that he is trying to promote with his story. If, for example, a gay character is presented as being the stereotypical flamboyant, femine type, then it's most likely that the person writing the story dissapproves of homosexuality and is attempting to convey that with the way he portrays his fictional, gay character.
However, if the gay character in question is portrayed in a different way, then it could be just as obvious that the author intends to express his or her belief that homosexuality is normal and acceptable and is using his book or TV show, or whatever as his means of trying to convey that message.
In this case, according to the article and Rawlings' own words, it seems that she made this character gay in an attempt to help teach the readers "tolerance", which translated means trying to influence her readers to embrace homosexuality.
So it's never so simple as just brushing it off as merely a fictional chracter in a book, whether were talking about a gay character or a character who is not gay. The stories people write are written with a certain values system in mind and are a means to express said values system and influence others to change their way of thinking. For example, one could write a story with fictional character who is a child molester. If the character gets his comeupance and pays the penalty for his actions, then it is obvious that the "moral of the story" is that child molestation is wrong. However, if the child molester is the hero of the story and is a well respected character, then obviously, the author is trying to say that it is okay to molest children and is using that book and that character as his means of changing public opinion regarding child molestation. I really doubt we'll ever see such a character, but I use that absurd hypothetical character to illustrate my point.
Having said that, I must admit that I am disturbed by this because I don't approve of homosexuality and I disagree with any attempt to portray it as normal and acceptable. I know I am in the minority and many others will disagree, and that's what a discussion board is all about: the exchange of ideas. But that is my opinion on the subject for whatever it's worth.