I believe the rainbow as the visible spectrum, to them, symbolizes illumination. They feel they are the enlightened ones, the ones with the greater knowledge, the ones with the greater wisdom, while we who stand against their perverted lifestyles are, to their view, the ones still in the darkness. It's a powerful symbolism. Add to that the fact that the colors for heterosexuality are black and white. So compare black and white to the rainbow, which is the gay symbol. Which one is more vibrant, more full of life, more attractive? It's another subtle form of brainwashing, that gay is good, that straight is limiting and boring. Rainbows of course also portray diversity, many different colors, while black and white are simply two opposite colors. I believe they like the rainbow to show that they are part of the human condition. In other words they want people to believe that they are simply a normal variation of what it means to be human and that homosexuality is not a sickness. I feel like these are the main reasons why they chose those colors.