I don't think anyone is encouraged into any field due to their gender. If someone notices you have a skill in an area then they will encourage you to pursue it.
Of course I think there are girls interested in it, I'm sure I said that in my reply. I think its great when they do. And I celebrate our society which makes it possible. Obviously we can always improve and do better as a society, but I think many people should acknowledge how far we have come as well.
I'm sorry that you didn't receive that encouragement, I suppose there may always be a cultural difference between the UK and America but over here I have observed nothing but encouragement for everyone pursuing anything.
And to your point about the boys reactions. I think men and women are wired differently, and they wouldn't respond like that because that's not the way they think. I'd argue that their "REALLY? YOU'RE in CS?" is their way of saying, "wow that's cool and unusual, good on you."