Is "guy" a disrespectful word?

Sometimes I face this issue in writing. I might be talking about a woman, but she's referred to as a girl often. It's a form of address that we're all familiar with though. Just remember that females mature faster than males, so while a boy is pretty much a boy until he's 18 (and sometimes through college) a girl might be a woman at as early as 12 in terms of physical maturity. My mother said she was pretty much fully grown at 15, and certainly looked older than a lot of those around her. So at what point does one transition into a woman? Also, a 'college girl' might be offended if you call them a 'college woman' which implies they look old. I think in this sense 'girl' is a youthful expression while 'woman' signifies age. It could fall in line with maturity in a different sense as well. An escort would be considered a 'working girl' and an office worker might be called a 'working woman'. In this case the implication is in terms of respect of position. Many other parallels could be made, but it's one of those weird connotations.

No, I can't see what's disrespectful about it. It's a casual, informal word but not a disrespectful one. It can be used for both men and women or mixed groups, for example a person might enter a room full of friends and call out 'Hi guys!' meaning everyone. Or say something like 'Come on guys, let's go out!'