Because of patriarchy and misogyny. Sometimes archaic attitudes harm men too.
Women are perceived as the weaker sex. She is not believed to have a will of her own. She is the property of her father then the husband. If she strays or makes mistakes, it is because her father or husband was unable to control her. She is not seen as having a mind of her own that is capable of doing good and bad things equally.
A recent horrific event due to this perception was that a teacher who sexually assaulted a child was released on bail because "she was a lady."
The laws have to evolve to include gender parity.
A child molester is a child molester, irrespective of gender.
An adulterer is an adulterer, irrespective of gender. (Although, I think adultery is grounds for divorce and should not be a legal or criminal offense)
The higher earning spouse pays alimony/child support, irrespective of gender.
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