It's not so much 'Spite' as the fact that, because society is usually so unaccepting of Homosexuality, once a chance comes along to openly accept that you are homosexual, some people get totally carried away.
In the case of politicians, they are often simply using the popularity generated by their comments for exposure, Jesse Jackson et al may be nuts, but you know who they are. That's more a case of Politics than true feeling, I think you'll find, if you're being offered too little, ask for too much, and what you'll get will be somewhere in the middle, and politicians are expert attention seekers. The NAACP and NOW etc also play on popular misconception and rumour, just as there are members of the gay community who will deliberately go out of their way to present themselves as a 'stereotypical' gay in Gay Pride Festivals etc, in many ways, it's a call for attention to what they have to say, some people find it offensive, but at least they are listening, even if it's only to decide it is offensive.
Edit: And yes, as Nuclear says, there are people out there who are genuinely angry, often these are the ones who have had to hide their homosexuality the longest, even from themselves, when they finally do come out, they do so somwhat extravagantly. And yes, the elimination of the need for 'Pride' would be great, but unlikely.