What does paying for pornography has to do with bitcoin. A knife maker can't be responsible that some people prefer to cut their wifes instead of bread. Most things in this world can be used to commit a crime.
Bitcoin has nothing to do with it, but people are stupid by nature. When people talk about Bitcoins on regular websites, they say that it's only used to buy drugs, and only criminals use it. It's the same type of logic that many people have when they see one member of one group do a horrible thing, they blame that whole group or country, etc.