In Carl's relationship guide, he refers to "relationship decay" between two friends (Building Friendship section) - that the relationship level naturally declines over time. However, when I was doing a relationship experiment game, I noticed that my Romantic Interest seemed to have a very, very slow relationship decay. I didn't buy the Long Distance Friend LTR either.
Has anyone else noticed this? If it's true, it would be a good way to speed up the rate your sims advance in a career: befriend coworkers and bosses, flirt, confess attraction, and then you can coast (Maybe!). Also, if you keep the coworkers only as RI I don't think it would affect any other relationships, romantic or otherwise.