This is probably related to a somewhat rare glitch where certain adult interactions are inadvertently unlocked by teens. Once, a long time ago, a teen sim I was playing was able to ask his girlfriend to move in with him. She did move in (on her own...no adult moved with her), but required her own separate bed. She wasn't able to sleep in a double bed with anyone in the household since they weren't related, and the option for them to get engaged/married didn't come up until they were both adults.
Your best bet is to simply uninstall and reinstall your game, and turn story progression off.
Your best bet is to simply avoid story progression at all costs.
This is a rather extreme "fix" for a simple issue, don't you think? I've only ever had this happen once in the entire time I've played the Sims 3, and played that particular household through several subsequent generations before abandoning it because I moved on to something else. Most people never see something like this. It's nowhere close to being a game breaker.
There is absolutely nothing wrong with story progression. Those who don't use it do not do so because of any bugs...they do so out of personal preference. But the majority of us bought Sims 3
because of story progression and have no significant problems
because of it.
