What’s Good for the Goose Is Good for the Gander

What does the idiom “What’s Good for the Goose Is Good for the Gander” mean?

Definition:

What’s OK for a man is OK for a woman, too

For example:

“My boyfriend complains when I go out, but then I found out he went out with his friends every night this week. What’s good for the goose is good for the gander.”

Notes:

This is rather old-fashioned. Still more old-fashioned is an earlier version, “what’s sauce for the goose is sauce for the gander.”