Interesting. I did a search on these forums looking for an answer to the same issue. I have two Sims that I pulled out of CAS and moved into separate homes. I'm trying a legacy where I only control one member of the family while everyone else is complete free will, and then pass the torch to each generation. So these two Sims met, became best friends, she moved in they've had two babies but NO ask to marry option pops up. I'm thinking these are fresh Sims who didn't have time to create relationships with anyone else. But now I think/wonder..... if they are pulled from the already made Sims, could they have pre-built relationships? I will have to check to see if one or both has a family tree (other than the two kids they currently have). Not something I would have thought to check!
Thanks,
rokrchik