Hi, Watashi is a standard word for "I", so men and women both can use it. Atashi is for girls use only, Ore is for boys. I'm not sure about now, but there are some girls who use Boku. I use it depending on situations, like when I'm talking among close friends. Or, when someone call me "Kun", then I use Boku.

i think its about politeness? ore is the most impolite one. as for the guy-girl thing, its like how some countries have words that are spoken by females n words by males e.g. thailand =] im not sure if tts e exact reason thou but tts how i tink of it.