In site settings, under SSL, you can define an SSL domain. That should take care of redirecting to the central SSL domain for the logon.

You should realize though, that only adding SSL for logins, and not for the rest of the authenticated session, is not a good security practice, and in fact offers fake security. The authentication cookie will still be transmitted over an unsecure connection, making it easy to hijack the cookie, and take over the session. If you want to offer your users real security, you will have to make sure that all authenticated sessions are secured with SSL.

I suspect the reason for just using SSL for one domain is that it used to be pretty hard to set up SSL for multiple domains in IIS, since you had to set up separate websites for that. Luckily, that has become much easier in Windows 2012 / IIS8.x, as you can now use Server Name Indication (SNI):  http://www.iis.net/learn/get-started/..., which allows you to use multiple different ssl certificates in one IIS website

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