I thought that it was what you wanted in fact
If you want your registered users to be able to login, while you still want non registered users to be able to purchase without creating an account, you can change the "registration" option to "no registration" as you already did. That way, no account will be created and the users won't have to enter a username and a password during the checkout. Note that of course, someone which already used his email address to register previously on your website won't be able to enter it for a "no registration" checkout since that email is already linked to a user account so he will have to login, or use another email address.