Suggestion: add a boyfriend/girlfriend/partner option to relationships in CAS
I noticed that this wasn't really an option when I was creating two young adults who I meant to be "fresh" in their relationship and new in town. And when you place that kind of household on the lot,...
You can already live together as you describe. Set them as Household members. Once you move in to a home you can quickly advance friendship and romance and they will live happily as an unmarried couple.