Cmv: i believe that the phrase "be a man" is at best outdated and at worst sexist.:
In my opinion, all it serves to do is remind men to fulfil the traditional gender roles. To "be a man" you have to provide for your family, be responsible and protect your family. It's the male equivalent of "be more ladylike". 

All it accomplishes is:  
* Make non-traditionally masculine men feel like shit   
* Reaffirm that men are the leaders and protectors of society   

It implies that these particular values are men-only, and that women aren't supposed to have those values (if they do, they're told to 'act like a lady'). And god forbid that men have some values that women are 'supposed' to have.

Why not have a gender neutral phrase? "Be a better person"?