I use MC Commander Center and with the pregnancy add-on i've seen a sim or 2 (just started playing again recently) get pregnant. With MC Command Center and the pregnancy module for it unplayed sims can get married and have children all on their own.
Without said mod though...they don't do anything except age. I love TS2 but I found TS4 just doesn't work like TS2 does, because played Sims will age without you if you play more than 1 household and I don't like that, so I play it like I do TS3, with just one family. To each their own, just saying the way I play. So with MC + Pregnancy module the neighborhood still feels alive with Sims getting married and having kids on their own. I have MC set to no culling and using the get_pop cheat to see the population I can do my own culling when I feel it's necessary, rather than letting the game take my Sims friends and partners and Sims I actually care about... :/
