It's not a political ideology or political speech. The reason we came up with freedom of speech is primarily so you can criticize the people in power. I know that people for whatever reason got this weird idea that pornography is somehow free speech, presumably because of lobbying, but historically free speech always coexisted with obscenity laws. If you want to silence your political enemy, it's going to be hard to do on the basis of pornography, but it's rather easy to do on a basis of 'hate speech'.