Why does it matter?
Most of my friends are part of a couple and do lots of couples nights out sometimes they decide to have a girls night out and the men go for a boys night out. I don't see why saying that is such a big deal. Men and women sometimes feel more relaxed/comfortable when out with just people of the same gender. Announcing it as such makes it clear that the opposite gender is not invited.
I wouldn't cry or get angry over the expression I would just choose not to use it. Some people can get dramatic over the most ridiculous things.