Humanists oppose sexism on principle
So certainly I oppose sexism since it is another form of prejudice, stereotyping, or discrimination, only this time based on ones gender. Both men and women can be victims of sexism. Men are expected to be the aggressive, rational, and dominant provider while women are the passive, intuitive, supportive nurturer.
It is impossible to really understand a human being if we judge them according to gender. But men see in woman, woman in men, too much of the others gender characteristics, and too little of what is individual. Gender stereotyping is a barrier to getting to know a unique individual. POF 14.3
Feminists oppose sexism on practical grounds
Feminists advocate for the rights of the underprivileged gender. The feminist movement fights for more opportunities for women because they are considered to be in the inferior position and need to be raised up to equality. Gender equality will be achieved in practical life by demanding change that improves the life of women.
Comparing feminism and humanism
On the practical side of gender rights, more social changes need to be made for woman, but this shouldn't prevent supporting the rights of men in areas where men face gender discrimination. Otherwise feminism would contradict itself by seeming to be a gender biased movement.
Each human nature is unique. Human beings must be allowed to decide for themselves what social roles are suited to their particular nature, and not be judged if these roles do not fit what others consider appropriate gender behavior. POF 14.4
Only an individual knows what lies within their own "natural disposition". Nonconforming gender people face discrimination, oppression, and violence for not adhering to society's traditional gender roles. That is why LGBT rights is a human rights issue.