Gay is a word with many meanings. A gay person is homosexual, but if we describe something like a scene or a party as gay, that means it's bright, merry, and happy.
Gay originally meant excited, festive and merry. Skipping down the street is something you'd do in a gay mood. A fun dinner party could be described as full of laughter and gaiety. Gay really only came to mean homosexual in the last century. The fight for gay rights doesn’t mean a fight for the right to be festive, it means a fight against discrimination.