Sims are still just as slutty as ever, but they can only be in one 'official' relationship - such as boyfriend/girlfriend, fiancee', spouse - at a time.  Other sims that are not in an official relationship with your sim have a red heart icon and are listed as "Romantic Interest."  So you can have a partner and the sim can still flirt/sleep around/whatever, but you can't ask another sim to go steady, etc. while the sim has a partner.  So they're not polygamous by design, but that doesn't mean they have to be monogamous.

Incidentally, in case anyone cares, I played my game again without the crash this time (but still after using the supercomputer to modify the relationship) and everything seems back to normal.  Other sims in that town have been able to develop relationships normally.  I downloaded CAW, too, but the problems I had started before I had downloaded it, so I don't think that's related.

I had the same problem in my game.  I think it is due to the sim having a 'stuck' relationship.  My sim asked to just be friends with her first girlfriend and neither were particularly upset at the time (which seemed strange) but then she couldn't go steady with later sims, regardless of traits/relationship levels/whether or not I controlled them.  I switched over to the household of one of her other lovers and asked if she was single, and the asking sim learned that she was in a relationship.  But it listed her partner as the first girlfriend and had the icon for romantic interest, which I don't think is supposed to count as a partner.  That sim was her romantic interest, but so were half a dozen others, so it was kind of weird.  I had her be just friends with the girlfriend again since break up wasn't listed as an option and this time they both had the broken heart moodlet for a while, but it didn't free up the 'go steady' interaction, and there was now no one that seemed to be her partner.

Based on this, I'm guessing that the first relationship stuck in at least some partial way.  I'm wondering if asking to just be friends instead of actually breaking up caused the original problem.  Is "just friends" an alternative to break up or is it supposed to occur after breakup, or what?  I'm not sure on that part. 

Ultimately, I couldn't fix her in any way with AM or in-game actions, so I ended up getting the supercomputer.  I've only used it once to make that sim and her current boyfriend "go steady" and it displayed the correct relationship, but about five minutes later my game began to hang and completely froze up my computer.  I haven't yet had a chance to play since then so that's as much as I know.  No idea if that's related or not, so downloader beware.  I'm also unsure if the change with the supercomputer is resetting the stuck part of my sims' relationship.  That sim wants to be a Heartbreaker so it'd be ideal if either she goes back to normal concerning relationships, or if the supercomputer will safely allow me to continue modifying them if they don't have the right type of relationship.  Seems like there's a bit of a workaround possible for this, but otherwise it's a frustrating problem.

Fieryhellpixie, the supercomputer is here: http://ts3.tscexchange.com/index.php?topic=1300.0