I found a discussion thread about this in the Sims 3 Forums, but the original article was from SimTimes.  Here is the article exert (loosely translated from German):

Did you create your Sims, you can in the relationship now, the options “Select Fixed friends Festivals friend” and “fiance / fiancee.

The just of the article is that in CAS you can make your sims go steady, be best friends or fiances.  This should be a great feature to use.