I wish that there was a setting in the relationship panel that you could set them as boyfriend or girlfriend or fiancée when you're creating a family. Especially I want them living together but there ...
You can with MCCC. But yeah, without a mod, would be nice if it was built into "create a sim."
My vote is putting this into "Base Game".
F.Y.I. Without mods, you can type in the cheat bar: cas.fulleditmode to edit relationships on existing households. It's basic, like brother, sister, mother, father, spouse... When a relationship is changed, keep in mind that the game automatically adjusts it to what makes sense.