Reasons to Avoid Sexist Language
Sexist language encourages discrimination and can discourage people from pursuing their dreams. If engineers are always spoken of as male, a girl who aspires to be an engineer may feel that she has no hope, since "all" engineers are men.
Sexist language also offends people when they find themselves excluded. This is not an issue that violates your right to free speech; you are free to use offensive language, and also free to decide that you do not agree with the definition of what constitutes sexist language. However, if you are using language that is offensive to half of your audience, you will not get your message across.