Do not send registration mail to fake users, who haven't specified their gender in registration form
I suggest you implement hook_form_id_alter to either add a new validation function to the registration form, or replace the submit callback.

In a validation function, you have access to the form values, and you can do whatever checks you want, and fail the submission. This will reject the form all together.

If you replace the submit callback instead, you could override only the sending of the e-mail. but otherwise let the form pass through, which seems closer to what is being asked for.