Once again, the errors people are making is that they're simplifying the problem too early. In probability you should always start by finding all possible outcomes and their probabilities, and only once you've done so can you consider the specific question. Here we have, from the outset, four equally likely outcomes (assuming boys or girls are equally likely) of GG, GB, BG, BB, of which one (GG) is excluded and one (BB) is what we're asked for. So it's 1/3 not 1/2 as GB and BG are two separate outcomes with the same probability.