For a long time, certain careers were only available to one gender or the other. Although that has changed, there are still careers that we tend to think of as more male or female; while this is primarily subconscious, it probably affects career choice a great deal.
Many of our career concepts are formed when we are quite young; a child who has not had the opportunity to meet a wide variety of people may develop narrow ideas about which careers "belong to" which genders. For instance, a child who has had only female teachers may think of teaching as a woman's job.
Many children are encouraged to act in ways that the adults around them think are gender-appropriate. For instance, girls may be encouraged to dress up and be less active physically, while boys are sometimes encouraged to hide fear or pain and act strong. This can lead to ideas about how one "should" act, which then translate to gender-biased ideas about careers. A woman who was taught to wear frilly clothes and keep them clean may avoid physical careers where she will get dirty. A man who was taught to hide his emotions may look upon careers in counseling or caregiving as inapproprate for males.