People see that "men and women are different" is used as justification for prejudice (like a person not getting a chance because people of the same sex did rarely have success previously). Which sucks and people feel its unfair. And when trying to conceptualize that unfairness into words, their contra opinion sometimes ends up being "so man and women must be equal" instead of "a persons sex does not justify prejudice".

Peoples feelings about fairness and justice are always valid, but hell, many people suck so hard at putting it into words.