When I play The Sims 3, I hate controlling large households of Sims - I prefer to play with a household of one, so I can focus all my attention on them without having to juggle tons of commands at once. The only problem with this is I like to make a Sim of myself - and obviously, I like my Sim to woohoo with a Sim of Shelly. See how faithful I am - I don't even like a virtual representation of me to cheat on a virtual representation of her. :)
Anyway, because I opt for the one Sim approach, my Sim is usually celibate - so I did some research to see if it was possible for CPU-controlled Sims to share a house with human-controlled ones. Long story short - yes, but it's awkward to set up and temperamental as hell.
Basically, you have to use a lot of console commands and the Sim had to start as a toddler, then age up to whatever life stage you wanted them to be - and you have very little control over their clothes/hairstyle/traits etc.
HOWEVER - after aging up child 'Shelly' Sim to a teenager, THIS happened:
Fuck it, she can just have a wank. :)