If you think it's just or not, it is still a double standard.
You're talking about the laws of various countries, I think you have Great Britain in mind.
That there is a more severe penalty for woman's adultery is
not a double standard. A woman who acts in a manner to make a man uncertain of the paternity of the children born to his wife is committing a graver offense than a philandering husband. An offense with graver consequences.
If a woman left her husband in the old days she was not required to support him.
Is that a "double standard" too?
When speaking of spousal support, there is no double standard in not requiring a husband to support an adulterous spouse.