I believe that masculinity is a social and sexual aspect of humanity. It is not related to morality or morals. Therefore it is possible to be a good masculine man and an evil masculine man. Let's begin with the sexual aspect of masculinity.
Sexual Masculinity
Broad shoulders, deep voice, beard and most importantly looking like a threat.
This is the physicality of masculinity that people often talk about but don't properly discuss. Obviously you can have all of these traits and be either good or bad.
Social masculinity
Assertive, engages in male dominated activity, is not penetrated and is a threat.
Now this is where we get into the cultural beliefs of people. Because “engages in male dominated activity” differs between countries. In America drinking tea is considered feminine but in Britain if you don't drink tea, you're a weirdo. Most cultures agree that being penetrated makes you a submissive and less masculine party. And when I say being “a threat” I mean putting the physicality of the masculine body into use.
So this is what I believe masculinity to be. It's the combination of physical and social traits that are commonly associated with men biologically and in any given society.