When I opened this thread, I thought I was going to be the only one whose sims usually stay single, or with the same person for all of eternity. But reading through the replies, it seems I'm in good company.
So yeah, most of my sims don't have a lot going on in their social lives, neither romantically nor platonically, because they tend to focus on other things. That's why the social bar gives me the most trouble when playing sims without the loner trait. If they do have a partner, it's usually the same one the entire time, and most spouses come out of CAS already married. Exceptions are, of course, sims with the Serial Romantic aspiration, but I've only tried that one two or three times so far and never completed it.
Another exception is that one sim whose biography identifies her as a black widow with many former husbands who died under mysterious circumstances. But between you and me, this is complete hogwash. I played this sim from birth to old age, and while she did have three babies from three different special NPCs (Father Winter, a Sulani elemental, and Patchy the scarecrow), she never actually married any of them and they're all still alive.
