This excellent post on Pandagon is a must-read about the way masculinity is put forth as both endangered and unfixed in American society. That is, men can lose their masculinity at any moment if they don't stay on guard... and men all around us are losing their masculinity. Oh my.
I recently had a bit of an email debate with some folks who will remain nameless about the propriety of insulting a man by calling him an a woman. The question was posed to another female who shared my views, "Are you insulted when someone calls you a man?" Ah, but that only proves the point. The idea behind the question was that of course you are insulted because you are a woman and by your nature abhor the idea of being masculine, and therefore it is a fair and proper insult in both directions. The catch is that no one ever insults a woman by calling her a man*... that label is saved for the times when you want to compliment a woman. So yes, you are insulted. But not because you've been called a man. Rather, because you've been told that your success makes you less of a woman.
* The pseudo-exception being lesbians, who are insulted for trying to be men. But I view this as yet another variation of the same theme.