Everyone throws other people into broad categories when they first meet. This isn't unusual; it's universal and that's why people make a big deal about "first impressions". You can't possibly know them when you first meet, but you can't wait until you know them better in order to react as you would if you did, so you slap a label on them based on past experience with first impressions and proceed. As you become more familiar with them your opinion of them gradually resolves from broad generalisations into specifics.

Most of the time we use broad everyman categories to begin with, but in particular circumstances (anything to do with mating especially) your first impression might be stronger as we've all had memorable (good or bad) experiences with people met at times like that. Since this impression influences your behaviour, powerful and especially negative impressions can be self-perpetuating.

The thing is you need to pretty much shut the fuck up and get to know people better before you act ignorantly. That'll equal your experiences out eventually, but keep in mind that your original negative impression had some sort of basis. A lot of the time that instinct you hate is probably right.