Prejudice is a (usually negative) preconceived belief, opinion, or judgment made without recourse to reason drawing typically instead upon received information or upon instinctual preference: any unreasonable attitude that is unusually resistant to rational influence.
Everyone has generalizations, because in life we have to make quick decisions based on limited available information and on our previous experiences. We rely on stereotypes to predict how people are likely to behave. Generalizations give us a certain security, a more or less reliable frame of reference.
But there comes a point beyond which generalizations become prejudice. This happens when you start to judge a group by the behaviour of its worst members. You collect anecdotes that reinforce your generalization and ignore those that don't. You dehumanize the group. You project opinions or attitudes such as superiority, aggression, selfishness, dishonesty or criminality on the group as a whole. You ignore or justify injustices or discrimination against the group, while resenting its privileges. You blame the group for its misfortunes. You criticise the way they look, the way they dress. You believe any misbehaviour of some member of the group means that all the rights and privileges of the group are automatically forfeit.
Anyone who is a member of a minority group will likely be familiar with this, and with the problems that prejudice causes. Cyclists probably have as many prejudices as anyone else, but like any minority group, they're particularly vulnerable to prejudices once they get established in the media and among less enlightened members of the population. You don't have to hang around the internet for long before you see it.
The good thing about cycling is that when you get off your bike and get changed, you're a regular person again. You're a husband/wife/partner, father/mother, son/daughter, worker, business person, customer, friend. In short, you're human again. The prejudiced can't get you any more.