Masculinity as properly defined is an aspirational and normative style of being and living as a natural-born man that a critical mass of the members of that population applaud. Masculinity may evolve over time and diverge within cultures, but there are trans-historical and trans-cultural aspects that any reasonable man can realistically point to through comparison using deitic adverbs. ("This is masculinity. That is not masculinity.") It is not a heteronormative phenomenon insofar as non-heterosexual males make the same denotations in whatever argot and the interactions between heredity and environment out of which the phenomenon of masculinity arises are visible in mass popular culture. As examples, James Bond, Sean Connery, Conan the Barbarian, and Arnold Schwarzenegger are masculine. When "masculine" is applied to women, the term denotes mannish features (or severe unattractiveness). That is not masculinity.
Woman: "That transsexual over there is quite masculine."
Man: "That is not masculinity."