Comment by UIUC_06

Just to complicate things:

One usage that is definitely single-sex is, one male might say to another male,

"Hi guy, how ya doin'?"

"OK, guy, let me tell you how to do this."

But he'd never say that to a female.

However, it wouldn't be that unusual to say to a whole group of women, whether one is male OR female,

"Hi guys, let's get started."