This proof is equally applicable to males. In fact, using proper logic, we come to an interesting, and I would say more complete, definition of reality.
Our initial assumption, that girls equal time and money, must be expanded. FOR BOYS, girls equal an OUTPUT of money, that is to say, there exists some sort of negative flux of money. We take time as positive for though the fact is that surely time is SPENT, there is only one direction in which time may flow (taken as positive). Our equation now reads:
Girls = (Time)(-Money)
Following, we acheive:
Girls = -(Money )( Money) = - (Money)^2
Girls = - (sqrt of evil)^2
Girls = -Evil
By our assumption, FOR GIRLS, boys equal an INPUT of money (or at least, material goods and/or services which have some finite positive value in currency) and (by our same assumptions for the nature of time) thus, for girls, boys = (time) (+money). We then:
Boys = (Time)(money)
And by the arguement delineated in the linked jpeg:
Boys = Evil
Thus we may, in fact, declare that in reality BOYS are evil, and girls are, in fact, the OPPOSITE (additive inverse, at least) of evil, which suggests girls are something along the lines of good, at least, nonevil in nature.
...Of course, this only goes to show that there is no such thing as a one-sided coin, and that perspective is everything; the original joke is, in fact, merely HALF of the real joke, so to speak. What the true conclusion ought to be is that humans are sometimes evil, sometimes good, and boys AND girls are human.
Kind of ruins the joke, though.
(furthermore, the two-sidedness comes full circle, as one can just as easily argue that girls equal an output of time on boys' parts, and boys, an output of time on girls' parts, and thus the negativities are reversed, eh?)