Well then you’re not my friend, guy.

“Guy” is used differently in different contexts, just like “girl”.

If someone says “the girls had a sleepover” you know they’re probably talking about female children. If someone’s talking about a girls’ night out, you know they’re almost certainly talking about women.

"Would you have sex with a group of guys?” is using guy as a stand-in for man. But, if someone said “Guys, can we all just quiet down for a second so Stan can speak?” it’s a synonym for “folks” or “people” or something gender-neutral.

Litmus test is the “Would you have sex with a group of guys?”

Dipping the tip into a group of guys and checking the color.