facts vs opinions

On the most basic level, a difference is a distinguishing characteristic, distinctive quality or feature, and a stereotpye is a conventional, formulaic, and oversimplified conception, opinion, or image.
While a stereotype is based on what someone thinks, a real difference is based on fact. Though we may not always use the most accurate words to express those differences, the idea that we are stating something that is true no matter who says it differentiates it from a stereotype which is based on perception. For example, calling African Americans "black" and Caucasians "white"... neither group is actually either of those colors, but the chosen words describe the factual difference between the tone of their skin. It just is. There is no bias or skew involved; black people's skin is darker than white people's. It becomes a stereotype rather than a difference when one begins to assume things based on these real differences. For example, if I were to say that all black people listen to rap music that would be a (n incorrect) stereotype; obviously this is a mild example, but it serves its purpose in showing that stereotypes take differences one step further.
As far as when marginalization is acceptable, I feel that differences in themselves marginalize the parties involved. This is, however, a necessarycomponent in separating and identifying various characteristics; it's not wrong because they just are. Stereotypes marginalize in an unacceptable manner in that they are often insulting. We must make sure to marginalize only according to differences as if we go by stereotypes we are being demeaning. We know that we are doing this only when we go by facts rather than opinioins.