As noted, people categorize somebody new by what they have dealt with in the past, by some shared characteristic.
Sometimes they're right, often they're wrong. The trick is realizing where your expectations lie, and knowing that you can be wrong. Will often be wrong. Note where they are, and don't let those expectations limit you.
Personally, people likely assume any number of things about me. My husband's company probably thinks I'm Christian; they assume everybody is. Anybody who hears me mention my husband will think I'm straight. I'm not. Actually, it's very odd (even after 9 years of marriage) for me to think I married a dude. I tend to use non-gendered pronouns whenever possible, and I know that has been noticed on occasion, with the following question in their minds of ... Why?
People see I'm blonde and will expect me to be dumb, innocent, or both. That's a fun game to play.
Some things you can use to judge a person- the punker is likely to hold a certain set of beliefs, and the guy with his head shaved is likely (in my area anyway) to be either a gang-banger or a White Supremecist. Not always true, of course, and I worked for a while with a guy who was shaved and absolutely the opposite end of the spectrum. Some appearance issues are chosen in order to broadcast membership in or identification with a particular group. Some are not.
I originally said I don't remember being stereotyped but I guess that was incorrect.