Using the word, gay

This class has really opened my eyes to the ignorance of people but this is one thing that has always bothered me. Why do people use the word gay to mean dumb or stupid? When I hear someone saying, "that's gay" when they are not talking about anything (even closely) related to being homosexual, I just want to scream at them. Why don't they understand how offensive that is? There are people who really are gay. Some people are still uncomfortable with sharing who they are and others are proud to express their identity but either way, this is offensive. How can people be so inconsiderate? Do I have a place to speak up? And who started this terrible phrase?