And if you asked them, "Hey guys, raise your hand if you use the web" you'd get men and women raising their hands. "the guys" != "guys" in this case. The word is very - or awfully, depending on your point of view - ambiguous, and can be used in a gender neutral/ambiguous way, like third-person plural pronouns in other languages.
Well, duh, that's because you used it in a non-gender-neutral way. If you said, "hey you guys, raise your hands", then that is way more gender neutral. Like all of language, differences in meanings can be subtle. They can also depend on where you are from.
If I address a mixed gender group, and I ask that "all the guys" raise their hand, everybody will know that I mean men only.