Unfortunately, even where it is common and normal language, "girl" to a woman holds implications that "guy" to a man doesn't. Specifically, that the woman is in

Unfortunately, even where it is common and normal language, "girl" to a woman holds implications that "guy" to a man doesn't. Specifically, that the woman is in a lesser position of power, since they're being referred to as a term that is used for young females, while the men are being referred to using a term which affirms their difference from young males. There's a point in the language that we use where boys with little power become guys with some power, but there isn't a point where girls with little power become [some word we don't really have outside of regional vernacular]. Our language is not perfect (even mine isn't) - the things we say on an everyday basis show all sorts of things we as a society implicitly believe, often without realising it. That our language reflects our society, and vice versa, is a very old idea, and part of modern feminism and the academia which backs it tries to bring that to the forefront.