You can change this in the controller by setting default value to 'guest' instead of register:
public_html/storefront/controller/pages/account/login.php:152
Change
'value' => (isset($this->session->data['account']) ? $this->session->data['account'] : 'register'),
To
'value' => (isset($this->session->data['account']) ? $this->session->data['account'] : ''),
and
'value' => ($this->session->data['account'] == 'guest' ? 'guest' : ''),
to
'value' => ($this->session->data['account'] == 'guest' ? 'guest' : 'guest'),