Is there any mod out there that allows for one to just play/control one sim and make the rest of the household unselectable? If one of my sims gets married, I would like for their husband/wife to be controlled like the townies and do stuff on their own, still living in the house with the sim but not being controlled by me at all. I'm curious.

Well, I don't know any mod that could do that but sometimes I just play it like that. I play with my self-sim and all other sims in the family do what they want. If you give your sims free will they will take care of themselves unless you chose other interactions for them. The children in my family even do their homework all alone. The worst thing that could happen is, that one of the other sims dies.

This is one of the reasons why I love the roommate system in Apartment Life. I like how they live together with us but still manage to tend their needs (well... sort of) without the player's interference.

MasterController together with MasterControllerCheats gives the option to make a Sim unselectable.

Look for ani's apartment mod here and create 2 separate families, one with your main sim and the other with the rest.

I don't think I could have uncontrollable sims in my household. All those radios coming on, all those computer games being played all at the same time...

..............................................................it's like being in a dorm at Uni

So does Debug Enabler. Just click on the Sim you don't want to control and go to Nraas > Debug Enabler > Options > Make Me a NPC. They will still be part of your household but will not be selectable.

Note: Be aware that travelling to vacation worlds, university or OL may cause problems if you bring along unselectable Sims, especially if they are ghosts. Example: I brought along an unselectable ghost on vacation and her thumbnail came back home but she did not. There was absolutely no way to recover her, either, without loading a pre-vacation backup save. You can still bring them along, but make sure you make them selectable again first before departing to and from sub-worlds. Once you've arrived, you should be safe to unselect them again.

If you've got University Life, you can use the phone to call for roommates (Real Estate>Roommates), but you won't have any control over who they are or what they look/act like without the use of Master Controller or Testing Cheats. If you don't have Uni (which I'm assuming you don't, but hey! Occam's razor, yo!) the mods listed above can help you get the same functionality, with some added control over the roommates. Plus you've got some options, so hey!