boku/watashi

A little question about Japanese style and grammar:

As far as I have learnt, when in first person, ぼく　is used by male speakers and わたし may be used both be males and females.
So, does using ぼく　mean emphasizing one's masculinity? Or does わたし represent a person as a non-sexist or just not being focused on gender matters? Or what?
Please explain.