You are totally right. I just changed it!
For many years I've been told as a non-native speaker from many native speakers, that "guys" is a common term to describe any group of people, no matter if women or men or both. I read it in books and heard it in TV shows and movies and that's pretty much how I learned to use that term.
I totally understand that it is offensive and want to unlearn it for quite a while. I'm sorry that it still sometimes slips into my sentences without me even noticing it. When English is not your first language it's quite hard to get to a point where you can just communicate without thinking about every word. Once you are there it feels even more difficult to me to unlearn some of the stuff again that took so long to learn. But I'm definitely aware of it being a problem and if you ever come across another place where I use it unintentionally, please let me know.