This thread is interesting because it's caught me at the point of beginning to identify less and less as a "girly boy" and more and more as genderqueer, something which is forcing me to rethink my own attitude towards a lot of things I previously had difficulty understanding. Obviously, I can't speak for all people who identify as genderqueer, it's a very diverse category (duh!) so let me speak for myself.
For a long time, I have not accepted the existence of a thing called gender identity. I have a very strong anti-identitarian streak when it comes to gender, in that I think not only is it bullshit that a person with one body part is assumed to be fundamentally different to a person with an inverted version of the same bodypart purely on the basis of the external appearance of their bodies. I also reject the notion that describing things as "manly" or "girly" is anything other than culturally specific delusions. The mere fact that a person can be a "girly boy" already illustrates (or should) that there's nothing to "girlyness" which is specific to girls.
There are two responses to this. One is apathy, which is the course I've always taken. It is to say that "well, gender doesn't matter so it doesn't matter what anyone thinks about me or how anyone interprets me". That's worked for a while, but lately it's not working out for me. After all, whether or not I accept that my gender doesn't mean anything doesn't mean anyone else feels the same. As anyone with a single gender studies class under their belts could tell you, we live in a world where gender is real. Typically, the first thing which is said when a baby emerges from the womb is whether it is a boy or a girl (or an "oh dear") and that single pronouncement then sets the course for its entire life. There is no choice in that, we don't get to speak up and say "um.. excuse me, I'm not sure I want you to treat me in this way based on this superficial observation of my body you have done now".
Thus, for me, identifying as genderqueer has been about reclaiming that right, even if it can only ever be partially realised. It's about refusing to give an answer to the question (which people will, amazingly, still come up and ask you in the street) of whether you're a boy or a girl, because however much you want to believe gender is meaningless that's a question you will constantly, constantly be called upon to answer through your actions, your behaviour, your dress, your body. There is no space in this world where it's possible for that answer genuinely, truly not to matter, but that doesn't mean it's pointless to try and create one, I think.
In short, being a "girly boy" means still accepting that the categories of "boy" and "girly" are real, and that they are authentic descriptions of myself and say something about me. I reject that point, and while calling myself "genderqueer" may be similarly meaningless, it functions as a necessary point of resistance in a system in which gender still is meaningful, whatever people might like to think.