The main idea behind this is that reasonable, intelligent people can disagree on politics. I think all the political prejudice comes into play when people with strongly held beliefs encounter people who disagree and are stupid. It just sets a bad precedent.
There's a special kind of irony in this matter for liberals: The basic idea behind political liberalism is that they're accepting and understanding of all creeds. They stand up for the People, oppressed and otherwise disenfranchised, but when it comes to opposing political beliefs, they won't even approach a conversation without their teeth bared. The kicker is that in a belief system in which subjectivism is king, they're absolutely opposed to many conservative beliefs.
It's not so hypocritical for conservatives--everyone know the only reason anyone would be conservative is if they're stupid or evil.1 At least they're consistent.
MESSAGE: Feel free to sex it up with liberals or conservatives, as long as they're not ugly. This doesn't apply to Libertarians and Green Party people.