Sexist language is language that is meant to include all people, but inadvertently excludes either men or women.
Why do writers need to avoid sexist language?
A writer wants to present a good ethos or character to the audience so the audience will find him or her believable. A good ethos will make your arguments even more convincing; a bad ethos will undermine your arguments by making your reader doubt your credibility.
If a writer uses language that the reader finds either offensive or inaccurate, the reader will lose faith in the writer's arguments. The end of all argumentation is persuasion, and you cannot persuade people when you offend them or make inaccurate claims.
How to avoid sexist language
1. Don't assume that a particular job is filled by a particular type of person: not only are many doctors women, but many nurses and secretaries are men. Use terms that can apply to both men and women. For example, the U.S. Post Office now officially calls the people who deliver the mail "mail carriers" rather than "mailmen." Talk about "flight attendants" instead of "stewardesses." Remember, the military is made up of both male and female soldiers, so a discussion about "the men in today's army" will exclude many soldiers.
2. Avoid gendered pronouns (like he and she, his and her, etc.) by making them plural. For example, the sentence "A nurse needs to care about her patients" can be changed to
"Nurses need to care about their patients."
3. Avoid terms like "man" when you mean to include women as well. "Man" (and similiar terms) can be confusing as well as inaccurate, since "man" might mean "all people" or just "male people." Change: "Since the beginning of time, man has worried about death," to "Since the beginning of time, people have worried about death."