Not marking the checkbox to receive email updates on the front end of a form is the same as actively Opting-Out of receiving Site Wide emails. If the front end user is interacting with your website for the first time, then a constituent record will be created with the Accept Email field populated with No. If a constituent record already exists for the email address on file, and the user is not logged in, and that record is currently Opted In or Out, then marking or unmarking the checkbox will have no effect on that record