I think the main reason why they have to be magical [b][i]girls[/i][/b] is because:

a) it's tradition for it to be girls. How many magic boy shows do you know exist?
b) the energy generated by emotions is higher if the [i]puella magi[/i] are young girls entering puberty. While boys may generate similar emotions, it's the girls who have higher energy output from said emotions - or at least how Kyubey put it.