Why is it that your partner’s parents make assumptions about you based on how you look, without getting to know you, and immediately think you are a bad person?
Prejudice. That’s a one-word answer, but not a very interesting one just on its own. The picture gets interesting when you look at it from a bird’s eye view and discover we all have prejudices about something or the other. Prejudice, i.e. preconceived opinions, either favourable or unfavourable of people or things, is a quick and dirty way that your brain stores information in the form of generalized ideas associated with particular symbols. For example, someone who wears a saree is more traditional than someone who wears a skirt, in India. It may or may not be true.
That’s the basis of most kinds of discrimination that go on in the world around us, whether it is based on our skin colours, nationalities, sexual orientations, accents and languages are spoken or the clothes are worn by people like you, me and practically anyone else.
It is the way our brain tries to detect and act on threat, from people and things that look different from us and our ways & values. It is not a very functional defence mechanism we have, but we have it. One only has to be aware of it and keep correcting oneself to not fall into the pitfalls of our nature. So we all have this “problem” but we don’t all choose to deal with it. So do take comfort in the fact that they are not singling you out and you may be doing that to others as well unwittingly.