I am not a guy.
Because I am not a guy, I hate it when people refer to members of my profession as "math guys." As in, "You should ask one of the math guys." Or "Those math guys sure are ugly."
"But Rebecca," you say. "Guy is a gender-neutral term."
No, it's not. If somebody walked into a room full of people and asked to speak to someone who possesses expert knowledge about optimization, supercomputing, and ulnar nerve entrapment, would you point in my general direction and say, "You need to talk to that guy over there, in the turquoise blouse"? No, you wouldn't, because I am not a guy. Guy is not a gender-neutral term, just like man is not a gender-neutral term.
Guys I can excuse when it is part of "you guys." That is an expression in the English language. Personally, I always say y'all, because I hate the term "guys," but I can overlook "you guys."
"But Rebecca," you say. "They are math guys. In this particular group, there actually aren't any women."
Fine, but you should call them "math folks" or "math group" or "math people" instead of "math guys" no matter the gender balance, unless you're trying to distinguish this particular group by using their gender to set them apart from other groups. While they may all be guys, possessing the "guy property" is not required in order to possess the "math property." Out of respect for the fact that those who are not guys could be members of the group, you should not use guys.
As a more obvious example, you could label them as the "math white people," because chances are good that they're all white. But you can easily see that "white people" is not necessary, because there are people of all races and ethnicities who are skilled in math. Like a race marker is not necessary in describing the group of mathematicians, a gender marker isn't needed either.
I hope that I have now convinced you, my vast readership, to use words such as folks, group, or people rather than guys when describing a collective group of people that may be composed of members of both sexes. If not, then please don't ever talk to me about groups of people, or I may feel inclined to puke on your shoes.