Another version of the saying is - "Whats sauce for the goose is sauce for the gander"
In other words - If its good enough for them, then its good enough for us.
Stating the obvious - a gander is a male goose but when cooked you wouldn't know whether it was male or female and the sauce is the same for each of them.
This saying can play on the male/female thing by inferring that if it's something he can go and do than so can she as eddiebun has already said. It's less to do with being good enough for both, rather being fair for both to do.