Wow, it's amazing how so many posts here completely forget about ... well about all of humany history. Yes, it is discriminatory to give girl coders a bonus. You know what else was discriminatory? Giving freed slaves 40 acres and a mule; it was absolutely unfair to say "white men, no mule for you!", but we did it anyway. How terribly unfair.
Just because something is discriminatory doesn't make it bad, and if you live in a fantasyland where you think history just goes out the window, and everyone is equal now so we should all just be treated exactly the same ... well then you live in a fantasy land. Come to the real world.
Now, that being said, there are often less discriminatory ways to fix past social injustices. Take affirmitive action: you can do it by race and be controversial, or you can do it by social class. If (say) African-Americans really are doing worse in society (as they are), they will be over-represented in the poorest social classes, and so a social-class based affirmitive action system would have the effect of benefiting (poor) African-Americans, without explicitly singling them out.
But it's not like Google can say "if you're a kid (of either gender), and you can see in to the future that you're not going to become a programmer, we'll give you $100". So in this case singling out girls is absolutely the right way to go, unless you think it's a good thing to have a highly desirable profession like programming VASTLY dominated by men.