I completely agree with your opinion. Yes. Its a sin.
But a person has a right to request/ask his wife at least once in life time. Not daily.
So, should take true permission from his wife before taking sanyasam.
if life mate does not allow to do so, then he does not have any right to leave his dependents, and should not leave his family.
Without life mate permission, if a male person takes sanyasam, then, it becomes a sin and treated/counted as evil karma, and he is forced to reincarnate again to experience and nullify such evil karma in the next life in the form of manglik defect or any other marital defect in the birth chart.
The same rule applies to a mother/father and a son.
e.g. Aadi Shankaracharya.
He took permission from his mother before taking sanyasam.
One should not leave dependents without their permission.