SFTP is not the same as FTPS. FTPS is FTP using SSL. SFTP is secure file transfer protocol (over SSH). (Mac OS X's FTP/FTPS support in Finder is pretty pathetic anyways.)
Mac OS X can not natively mount SFTP shares. There is a way to do it using MacFUSE. It's rather difficult to set up and there's not a reliable non-commercial GUI application to manage it. There are a couple packages you have to install and a lot of command line hackery to get shares to mount (don't get me started on auto mounting them). It's really not worth the effort. I have done it a couple times before, but it was hell; I have found a much better solution: ExpanDrive.
It's a commercial program but it's worth every penny. It uses MacFUSE and allows you to mount SFTP, FTP/FTPS, and Amazon S3 "drives". You can download a free trial which works for 30 days. (Also, it won't expire unless you close it. :))
In order to get it to work using public key pairs you'll need to add your SSH private key to your Mac OS X keychain:
ssh-add -K Then when you create a new drive in ExpanDrive leave the password field blank, but check the "Save" box.
You should be good to go. The great thing about ExpanDrive is it will auto reconnect the drives if the connection gets dropped (wifi drops, you sleep/wake your mac, you reboot).