I agree with Mikezumi in that a Sims action will tell what they want and switching households will break player guidance, if it is not in line with what the Sims want. Also note, that some actions are guided (forced) by story progression, EA's version is known to stomp all over player efforts after they make a household inactive; it doesn't care about what the Sim wants it just fulfills a mathematical equation with no rhyme or reason.
However, if both you and your Simmie want the same thing, they will continue on that path whether you are there to guide them or not. They will continue trying even if SP has created obstacles for them (other than killing one of them).
There's also the little known fact, although Sims are all digital beings, they do not operate at the same 'speed' if you will. Some Sims are slow learners, while others are quick to pick things up. It all depends on the Sim and we have no control over that aspect of them.
A slower set Sim will appear to move in a way which can appear to be uninterested, cautious or just plain slow. When coupled with the 'wish triggering system' of the game, it can give the illusion things have slowed down for a Sim who was only a short time ago gun-ho for the romance. This isn't the case at all. It just takes a slower set Sim a longer amount of time to process information and make decisions which will affect their lives and their wants for self. Rosalie may just be considering all aspects of her relationship to develop a feel for if this is truly an acceptable fit for her.