Well, certainly not always. It will depend on who you are calling "guys". The younger crowd seems to accept it, but from my experiences the older do not.
Or it could be regional. I recall one time after I started a new job some 20 years ago, one of the senior members of the staff, a man in his 50s, used the phrase "you guys" when addressing several other coworkers, both male and female, all younger than him. A discussion ensued about whether or not women could be considered part of the "guys". It was a light-hearted debate, and the consensus was that women could indeed be included thus.
Sorry for the topic derailment