I suspect a more realistic scenario is that it's the other girls who do the mocking and rejecting as a reaction to one person's interest in computer science.. that's the thing.

the question then becomes.. why does the little girl care what the other little girls think ? Why isn't she more than happy hanging out with the more open and accepting group of computer cherishing boys ?

I assume it has to do with social norms, early childhood developmental experiences, not wanting to break certain social guidelines passed down through family values, the anticipation of "dating"..