Yeah, I like breeding my sims too, to see what the children of different combinations will turn out like. ;) My snob has two children with her boyfriend now, but she was cheating on him with my crazy redhead when she had the second. I was very tempted to have her second baby with the redhead, and see what her boyfriend does if the baby has red hair. Would a sim figure that out?
But now after having 2 kids with the same dad, she doesn't accept the redhead's advances anymore (and he doesn't accept hers either). Does having more than one child with a sim make them more loyal to their mate, or did they change the game mechanics to make characters less slutty ?