Source The Manual: What Women Want and How to Give It to Them
Most people seem to believe that what constitutes feminine and masculine behavior is dynamic and may change over time as the behavior of the majority of a sex changes. Some scholars even list examples from history of such changes, but they mistake expressions of femininity for femininity itself and expressions of masculinity for masculinity itself. The majority of males do not have to adopt a trait for it to become a masculine trait. In fact, many males, perhaps even the majority these days, are more timid than brave, but being timid will never be considered masculine because the majority does not set the standards. Biology does, and to be timid is more a sign of vulnerability than bravery; hence, it is more suitable for females, at least in our natural environment where they were more likely to survive and reproduce if they avoided risks.
I don't understand the expressions "mistake expressions of femininity itself" and "expressions of masculinity for masculinity itself", and "adopt a trait for it to become a masculine trait", so the sentences is hard for me to understand. What does the writer mean in this highlighted sentences?