Generalizations are an essential part of human knowledge, and we use them all the time when we interact with reality. Our brain cannot really work without it.
Unfortunately they are not accurate and from the single person point of view they are called "prejudices" and are most annoying.
Most of the times we do not even bother to generalize ourselves. We get the generalizations prêt-à-porter from the mainstream culture of our society or our social group, and we prolong the life of incomprehensions that have their roots in a far away past.
Gender, nationality, race and social class prejudices have a huge impact on our life. Sometimes they are unbearable and outrageous. Sometimes they become self-fulfilling prophecies.

But... if we cannot think without them, it is naive to pretend to get rid of them as a whole. We have to destroy them one at the time, knowing that like a phoenix they regenerate every time.