Hi,

Update your code like this,

<field name="marital"/>
<field name="marital_id" invisible="1"/>
<field name="wedding_annniv" attrs="{'invisible':[('marital_id', '!=', True)]}"/>
<field name="spouse_name" attrs="{'invisible':[('marital_id, '!=', True)]}"/>
<field name="spouse_dob" attrs="{'invisible':[('marital_id', '!=', True)]}"/>

In the Python,

marital = fields.Selection([
    ('single', 'Single'),
    ('married', 'Married'),
    ('cohabitant', 'Legal Cohabitant'),
    ('widower', 'Widower'),
    ('divorced', 'Divorced')], string='Marital Status', default='single')
marital_id = fields.Boolean(string='Status')

@api.onchange('marital')
def _marital(self):
    if self.marital and self.marital == 'married':
        self.marital_id = True
    else:
        self.marital_id = False

Thanks