It's because it's been scorned for thousands of years to be gay. People are proud that they can finally be openly gay and not get lynched or burned at the stake. It may be irritating, but you have to realize that these people have been told for their entire lives that their sexuality is not okay - that's its wrong, immoral, disgusting, etc. etc.
I think it's alright to let someone show their pride when they've been told to be ashamed of it so intensively.