Because it's generally a double standard, in that the same thing is not expected of men. It goes along with the 'women need to look attractive in the workplace' thing.
Men will get the "man up", "grow a pair", "relax", "you'll get over it", "are you gonna pussy out on me?" etc.
It's a hard problem --and unless you go for rigidity in the workplace, we're always going to have regular human issues (not company issues) permeate the workplace --be it office romance, cajoling, hijinks, etc.